feat(generator): delegate REST transcoding to google-api-core

[hebaalazzeh]

this PR refactors generator templates to delegate REST transcoding to the centralized helper in google-api-core.

Scope of Changes:

• _shared_macros.j2: Replaced inline static helper invocation with gapic_v1.rest_transcoding.transcode().
• rest_base.py.j2: Stripped the redundant _get_transcoded_request, _get_request_body_json, and _get_query_params_json helper functions.
• _rest_mixins_base.py.j2: Stripped the redundant static helpers from API REST mixins base.
• rest.py.j2 & rest_asyncio.py.j2: Passed the configuration flags (like opts.rest_numeric_enums) into the transcoding macro.
• noxfile.py: Configured local packages/google-api-core to be installed during virtualenv test generation so the new transcoding runtime module is available.

related PR: #17765

[gemini-code-assist]

Code Review

This pull request refactors REST transcoding in the GAPIC generator by centralizing the transcoding logic into a new transcode function within google-api-core (gapic_v1.rest_transcoding). This change eliminates redundant static helper methods (_get_transcoded_request, _get_request_body_json, and _get_query_params_json) from the generated transport templates. Additionally, the PR updates test configurations and introduces comprehensive unit tests for the new transcoding utility. The feedback suggests optimizing the transcode function by using json_format.MessageToDict directly instead of the inefficient round-trip serialization of json.loads(json_format.MessageToJson(...)).

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[hebaalazzeh]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces a compatibility module (_compat.py.j2) to support older versions of google-api-core in generated clients, replacing local helper methods in transport templates with centralized calls to rest_helpers.transcode_request. Additionally, the local development setup in noxfile.py is updated to install google-api-core from source. Feedback on the changes highlights two critical issues: first, the use of getattr to retrieve __REQUIRED_FIELDS_DEFAULT_VALUES in _shared_macros.j2 will fail at runtime due to Python's name mangling, requiring the use of the mangled name; second, in _compat.py.j2, older versions of google-api-core might have transcode_request but lack the rest_numeric_enums parameter, which would cause a TypeError unless the function signature is inspected first.

[daniel-sanche]

It would be helpful to see the generated code here. Reviewing raw templates alone can be tricky