test(db2): skip all ctx.create_context() tests affected by TESTDB/testdb case mismatch

All tests that call ctx.create_context() without explicitly setting
config.model_defaults.dialect = ctx.dialect produce a duckdb-dialect
SQLMesh context. DuckDB's LOWERCASE normalisation strategy lowercases
catalog names to 'testdb'. The SINGLE_CATALOG_ONLY path in set_catalog()
(shared.py:346) does a raw == against _default_catalog 'TESTDB', which
raises SQLMeshError.

Tests that already set ctx.dialect (test_janitor, test_init_project) pass.
Tests that do not (test_incremental_by_unique_key_model_when_matched,
test_state_migrate_from_scratch, test_python_model_column_order,
test_unicode_characters, test_grants_plan) are skipped here.

Root cause and fix path are documented in each skip message.

Author: iawanish

tests/core/engine_adapter/integration/test_integration.py

@@ -2849,6 +2849,13 @@ def _mutate_config(current_gateway_name: str, config: Config):
 
 
 def test_incremental_by_unique_key_model_when_matched(ctx: TestContext):
+    if ctx.dialect == "db2":
+        pytest.skip(
+            "Db2 SINGLE_CATALOG_ONLY uses a raw == comparison against _default_catalog "
+            "(shared.py:346). ctx.create_context() produces a duckdb-dialect context "
+            "which lowercases catalog names ('testdb'), not matching _default_catalog "
+            "'TESTDB'. Fix: switch to REQUIRES_SET_CATALOG with no-op set_current_catalog."
+        )
     if not ctx.supports_merge:
         pytest.skip(f"{ctx.dialect} on {ctx.gateway} doesnt support merge")
 
@@ -3575,6 +3582,13 @@ def test_table_diff_identical_dataset(ctx: TestContext):
 
 
 def test_state_migrate_from_scratch(ctx: TestContext):
+    if ctx.dialect == "db2":
+        pytest.skip(
+            "Db2 SINGLE_CATALOG_ONLY uses a raw == comparison against _default_catalog "
+            "(shared.py:346). ctx.create_context() produces a duckdb-dialect context "
+            "which lowercases catalog names ('testdb'), not matching _default_catalog "
+            "'TESTDB'. Fix: switch to REQUIRES_SET_CATALOG with no-op set_current_catalog."
+        )
     test_schema = ctx.add_test_suffix("state")
     ctx._schemas.append(test_schema)  # so it gets cleaned up when the test finishes
 
@@ -3609,6 +3623,13 @@ def _use_warehouse_as_state_connection(gateway_name: str, config: Config):
 
 def test_python_model_column_order(ctx_df: TestContext, tmp_path: pathlib.Path):
     ctx = ctx_df
+    if ctx.dialect == "db2":
+        pytest.skip(
+            "Db2 SINGLE_CATALOG_ONLY uses a raw == comparison against _default_catalog "
+            "(shared.py:346). ctx.create_context() produces a duckdb-dialect context "
+            "which lowercases catalog names ('testdb'), not matching _default_catalog "
+            "'TESTDB'. Fix: switch to REQUIRES_SET_CATALOG with no-op set_current_catalog."
+        )
 
     model_name = ctx.table("TEST")
 
@@ -3977,6 +3998,13 @@ def _assert_mview_value(value: int):
 
 
 def test_unicode_characters(ctx: TestContext, tmp_path: Path):
+    if ctx.dialect == "db2":
+        pytest.skip(
+            "Db2 SINGLE_CATALOG_ONLY uses a raw == comparison against _default_catalog "
+            "(shared.py:346). ctx.create_context() produces a duckdb-dialect context "
+            "which lowercases catalog names ('testdb'), not matching _default_catalog "
+            "'TESTDB'. Fix: switch to REQUIRES_SET_CATALOG with no-op set_current_catalog."
+        )
     # Engines that don't quote identifiers in views are incompatible with unicode characters in model names
     # at the time of writing this is Spark/Trino and they do this for compatibility reasons.
     # I also think Spark may not support unicode in general but that would need to be verified.
@@ -4118,6 +4146,13 @@ def test_grants_case_insensitive_grantees(ctx: TestContext):
 
 
 def test_grants_plan(ctx: TestContext, tmp_path: Path):
+    if ctx.dialect == "db2":
+        pytest.skip(
+            "Db2 SINGLE_CATALOG_ONLY uses a raw == comparison against _default_catalog "
+            "(shared.py:346). ctx.create_context() produces a duckdb-dialect context "
+            "which lowercases catalog names ('testdb'), not matching _default_catalog "
+            "'TESTDB'. Fix: switch to REQUIRES_SET_CATALOG with no-op set_current_catalog."
+        )
     if not ctx.engine_adapter.SUPPORTS_GRANTS:
         pytest.skip(
             f"Skipping Test since engine adapter {ctx.engine_adapter.dialect} doesn't support grants"