feat(generator): gapic generator centralization routing

[hebaalazzeh]

This PR refactors the generated client templates to utilize centralized endpoint routing and universe domain logic.

• Helper Relocation: Moved _get_default_mtls_endpoint, _get_api_endpoint, and _get_universe_domain from generated client classes to the compatibility layer (_compat.py).
• Backward Compatibility: Implemented these helpers locally in _compat.py to maintain compatibility with older google-api-core versions, laying groundwork for importing directly from google.api_core.universe (centralized in PR feat(api-core): add get_universe_domain helper to universe.py #17799) once the minimum dependency is bumped.
• Test Migration: Moved unit tests for these routing functions from test_%service.py.j2 to test_compat.py.j2.

Towards: #17883

[gemini-code-assist]

Code Review

This pull request introduces a new compatibility template _compat.py.j2 to provide fallback implementations for get_default_mtls_endpoint, get_api_endpoint, and get_universe_domain when they are not available in google.api_core.universe. The client templates are updated to delegate these operations to the new compatibility module, and redundant unit tests are removed from the generated test suites. The review feedback highlights that both get_api_endpoint in _compat.py.j2 and _get_api_endpoint in client.py.j2 can return None under certain conditions, and recommends updating their return type annotations to Optional[str] for better type safety.

[hebaalazzeh]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces a compatibility module template _compat.py.j2 to centralize fallback logic for older versions of google-api-core, updating the generator and client templates to import and use these shared utilities. Feedback on the changes highlights two critical issues regarding generated files: first, the new mock.patch("os.path.exists", ...) in the golden files is missing from the generator's template test_%service.py.j2; second, the new test file test__compat.py was added directly to a golden directory without a corresponding template, which will cause integration test failures.