test(langchain): activate test_responses_spec tests

[cbornet]

description by @mdrxy

• Enable test_responses_spec.py integration tests that were previously skipped at module level
• Widen ToolStrategy.schema type annotation from type[SchemaT] to type[SchemaT] | dict[str, Any] to match actual supported usage (JSON schema dicts were already handled at runtime)
• Fix type annotations and linting issues in test file (modernize to dict/list, add return types, prefix unused _request param)
• Improve generic typing in load_spec utility with bounded TypeVar