feat(generator): emit resumable upload settings and HttpJson upload stub

Adds generator support for protocol-specific resumable upload options, independent of any transport stub wiring:

- Emit ResumableUploadCallSettings on the generated <Service>Settings and
  <Service>StubSettings, populated with global timeouts extracted from the
  service config while bypassing standard unary retries.
- Add HttpJsonServiceResumableUploadStubClassComposer, which generates the
  dedicated REST stub (HttpJson[Service]ResumableUploadStub) for services
  containing resumable upload RPC methods.
- Extract HttpJsonDescriptorComposer out of HttpJsonServiceStubClassComposer
  and add path prefix support. Pure refactor with no golden changes, reused by
  the upload stub composer above.

Supersedes #14318 and #14320, which are folded in here.

Author: whowes

sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/comment/SettingsCommentComposer.java

@@ -78,6 +78,10 @@ public class SettingsCommentComposer {
   public static final CommentStatement DEFAULT_CREDENTIALS_PROVIDER_BUILDER_METHOD_COMMENT =
       toCommentStatement("Returns a builder for the default credentials for this service.");
 
+  public static final CommentStatement SET_HTTP_JSON_INTERNAL_HEADER_PROVIDER_METHOD_COMMENT =
+      toCommentStatement(
+          "Sets the internal HeaderProvider for HTTP/JSON. Does not change the transport.");
+
   public static final CommentStatement DEFAULT_TRANSPORT_PROVIDER_BUILDER_METHOD_COMMENT =
       toCommentStatement("Returns a builder for the default ChannelProvider for this service.");
 
@@ -130,6 +134,16 @@ public static CommentStatement createCallSettingsGetterComment(
         isMethodInternal);
   }
 
+  private static final String RESUMABLE_UPLOAD_CALL_SETTINGS_DOC_NOTE =
+      "Note that the settings configured here apply strictly to the initial session initiation"
+          + " request, and not to the subsequent data transfer requests.";
+
+  public static CommentStatement createResumableUploadCallSettingsGetterComment(
+      String javaMethodName, boolean isMethodDeprecated, boolean isMethodInternal) {
+    return createResumableUploadCallSettingsComment(
+        CALL_SETTINGS_METHOD_DOC_PATTERN, javaMethodName, isMethodDeprecated, isMethodInternal);
+  }
+
   public static CommentStatement createBuilderClassComment(String outerClassName) {
     return toCommentStatement(String.format(BUILDER_CLASS_DOC_PATTERN, outerClassName));
   }
@@ -140,6 +154,30 @@ public static CommentStatement createCallSettingsBuilderGetterComment(
     return toCommentStatement(methodComment, isMethodDeprecated, isMethodInternal);
   }
 
+  public static CommentStatement createResumableUploadCallSettingsBuilderGetterComment(
+      String javaMethodName, boolean isMethodDeprecated, boolean isMethodInternal) {
+    return createResumableUploadCallSettingsComment(
+        CALL_SETTINGS_BUILDER_METHOD_DOC_PATTERN,
+        javaMethodName,
+        isMethodDeprecated,
+        isMethodInternal);
+  }
+
+  private static CommentStatement createResumableUploadCallSettingsComment(
+      String pattern, String javaMethodName, boolean isMethodDeprecated, boolean isMethodInternal) {
+    JavaDocComment.Builder docBuilder =
+        JavaDocComment.builder()
+            .addComment(String.format(pattern, javaMethodName))
+            .addParagraph(RESUMABLE_UPLOAD_CALL_SETTINGS_DOC_NOTE);
+    if (isMethodDeprecated) {
+      docBuilder.setDeprecated(CommentComposer.DEPRECATED_METHOD_STRING);
+    }
+    if (isMethodInternal) {
+      docBuilder.setInternalOnly(CommentComposer.INTERNAL_ONLY_METHOD_STRING);
+    }
+    return CommentStatement.withComment(docBuilder.build());
+  }
+
   public static List<CommentStatement> createClassHeaderComments(
       String configuredClassName,
       String defaultHost,

sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/common/AbstractServiceSettingsClassComposer.java

@@ -25,6 +25,7 @@
 import com.google.api.gax.rpc.ClientSettings;
 import com.google.api.gax.rpc.OperationCallSettings;
 import com.google.api.gax.rpc.PagedCallSettings;
+import com.google.api.gax.rpc.ResumableUploadCallSettings;
 import com.google.api.gax.rpc.ServerStreamingCallSettings;
 import com.google.api.gax.rpc.StreamingCallSettings;
 import com.google.api.gax.rpc.StubSettings;
@@ -153,7 +154,7 @@ private static List<CommentStatement> createClassHeaderComments(
     // list.
     List<Method> publicMethods =
         service.methods().stream()
-            .filter(m -> m.isInternalApi() == false)
+            .filter(m -> !m.isInternalApi() && !m.isResumableUpload())
             .collect(Collectors.toList());
     Optional<Method> methodOpt =
         publicMethods.isEmpty()
@@ -311,12 +312,18 @@ private static List<MethodDefinition> createSettingsGetterMethods(
   // Add method header comment statements and annotations.
   private static MethodDefinition methodBuilderHelper(
       Method protoMethod, MethodDefinition.Builder methodBuilder, String javaMethodName) {
-    return methodBuilder
-        .setHeaderCommentStatements(
-            SettingsCommentComposer.createCallSettingsGetterComment(
+    CommentStatement commentStatement =
+        protoMethod.isResumableUpload()
+            ? SettingsCommentComposer.createResumableUploadCallSettingsGetterComment(
+                getMethodNameFromSettingsVarName(javaMethodName),
+                protoMethod.isDeprecated(),
+                protoMethod.isInternalApi())
+            : SettingsCommentComposer.createCallSettingsGetterComment(
                 getMethodNameFromSettingsVarName(javaMethodName),
                 protoMethod.isDeprecated(),
-                protoMethod.isInternalApi()))
+                protoMethod.isInternalApi());
+    return methodBuilder
+        .setHeaderCommentStatements(commentStatement)
         .setAnnotations(createMethodAnnotations(protoMethod))
         .build();
   }
@@ -794,13 +801,19 @@ private static List<MethodDefinition> createNestedBuilderSettingsGetterMethods(
       String javaMethodName = String.format("%sSettings", javaStyleName);
       MethodDefinition.Builder methodBuilder =
           methodMakerFn.apply(getCallSettingsBuilderType(protoMethod, typeStore), javaMethodName);
+      CommentStatement commentStatement =
+          protoMethod.isResumableUpload()
+              ? SettingsCommentComposer.createResumableUploadCallSettingsBuilderGetterComment(
+                  getMethodNameFromSettingsVarName(javaMethodName),
+                  protoMethod.isDeprecated(),
+                  protoMethod.isInternalApi())
+              : SettingsCommentComposer.createCallSettingsBuilderGetterComment(
+                  getMethodNameFromSettingsVarName(javaMethodName),
+                  protoMethod.isDeprecated(),
+                  protoMethod.isInternalApi());
       javaMethods.add(
           methodBuilder
-              .setHeaderCommentStatements(
-                  SettingsCommentComposer.createCallSettingsBuilderGetterComment(
-                      getMethodNameFromSettingsVarName(javaMethodName),
-                      protoMethod.isDeprecated(),
-                      protoMethod.isInternalApi()))
+              .setHeaderCommentStatements(commentStatement)
               .setAnnotations(createMethodAnnotations(protoMethod))
               .build());
 
@@ -856,6 +869,7 @@ private static TypeStore createStaticTypes() {
             Operation.class,
             OperationCallSettings.class,
             PagedCallSettings.class,
+            ResumableUploadCallSettings.class,
             ServerStreamingCallSettings.class,
             StreamingCallSettings.class,
             StubSettings.class,
@@ -933,7 +947,13 @@ private static TypeNode getCallSettingsTypeHelper(
       Method protoMethod, TypeStore typeStore, boolean isBuilder) {
     Class<?> callSettingsClazz =
         isBuilder ? UnaryCallSettings.Builder.class : UnaryCallSettings.class;
-    if (protoMethod.isPaged()) {
+    if (protoMethod.isResumableUpload()) {
+      return TypeNode.withReference(
+          ConcreteReference.withClazz(
+              isBuilder
+                  ? ResumableUploadCallSettings.Builder.class
+                  : ResumableUploadCallSettings.class));
+    } else if (protoMethod.isPaged()) {
       callSettingsClazz = isBuilder ? PagedCallSettings.Builder.class : PagedCallSettings.class;
     } else if (protoMethod.isBatching()) {
       callSettingsClazz =