fix(@angular/build): inline external sourcemaps for workspace library files

When no Babel plugins are required, the JavaScript transformer returns library
files as-is, preserving the  comment but never reading
the referenced map file from disk. esbuild does not follow external sourcemap
links in input files, so the chain from bundled output back to the original
TypeScript source is never formed.

Read the external map file and return an inline base64 sourcemap instead.
esbuild processes inline sourcemaps from input files correctly, allowing it
to compose the full sourcemap chain through to the original TypeScript source.

Author: JSMike

packages/angular/build/src/tools/esbuild/javascript-transformer-worker.ts

@@ -95,7 +95,43 @@ async function transformWithBabel(
   // If no additional transformations are needed, return the data directly
   if (plugins.length === 0) {
     // Strip sourcemaps if they should not be used
-    return useInputSourcemap ? data : data.replace(/^\/\/# sourceMappingURL=[^\r\n]*/gm, '');
+    if (!useInputSourcemap) {
+      return data.replace(/^\/\/# sourceMappingURL=[^\r\n]*/gm, '');
+    }
+
+    // Inline any external sourceMappingURL so esbuild can chain through to the original source.
+    // When no Babel plugins run, external map references are preserved in the returned data but
+    // esbuild does not follow them. Converting to an inline base64 map allows esbuild to compose
+    // the full sourcemap chain from bundle output back to the original TypeScript source.
+    const externalMapMatch = /^\/\/# sourceMappingURL=(?!data:)([^\r\n]+)/m.exec(data);
+    if (externalMapMatch) {
+      const mapRef = externalMapMatch[1];
+      const fileDir = path.dirname(filename);
+      const mapPath = path.resolve(fileDir, mapRef);
+      // Reject path traversal — the resolved map file must remain within the source
+      // file's directory tree and must be a .map file. This prevents a crafted
+      // sourceMappingURL from reading arbitrary files from disk.
+      const fileDirPrefix = fileDir.endsWith(path.sep) ? fileDir : fileDir + path.sep;
+      if (!mapPath.startsWith(fileDirPrefix) || !mapPath.endsWith('.map')) {
+        return data;
+      }
+      try {
+        const mapContent = await fs.promises.readFile(mapPath, 'utf-8');
+        const inlineMap = Buffer.from(mapContent).toString('base64');
+        return data.replace(
+          /^\/\/# sourceMappingURL=[^\r\n]*/m,
+          `//# sourceMappingURL=data:application/json;charset=utf-8;base64,${inlineMap}`,
+        );
+      } catch (error) {
+        // Map file not readable; return data with the original external reference
+        // eslint-disable-next-line no-console
+        console.warn(
+          `Unable to inline sourcemap for '${filename}': ${error instanceof Error ? error.message : error}`,
+        );
+      }
+    }
+
+    return data;
   }
 
   const result = await transformAsync(data, {