Fix Pydantic v2 compatibility and address PR review feedback

- Fix critical bug in GenericAlias handler calling (handler is callable, not object with generate_schema method)
- Completely rewrite test_pydantic_v2_compatibility.py to address all PR feedback:
  - Remove invalid imports of nested functions (__get_pydantic_core_schema__)
  - Fix assertion mismatches (OpenAPI version 3.1.0, correct API title)
  - Restructure tests to validate patching mechanism itself
  - Fix handler mocking to treat handlers as callables
  - Add comprehensive behavior tests with proper classmethod calling
- All 17 tests now pass with full coverage of patching scenarios
- Address types.GenericAlias immutability issues in tests
- Improve error handling and fallback validation

Author: mmphego

src/google/adk/utils/pydantic_v2_compatibility.py

@@ -76,7 +76,7 @@ def generic_alias_get_pydantic_core_schema(cls, source_type, handler):
             if hasattr(source_type, "__origin__") and hasattr(source_type, "__args__"):
                 try:
                     # Let pydantic handle the origin type (list, dict, etc.)
-                    return handler.generate_schema(source_type.__origin__)
+                    return handler(source_type.__origin__)
                 except Exception:
                     # Fallback to any schema if we can't handle the specific type
                     return core_schema.any_schema()