feat(api-core): centralize rest transcoding helpers

[hebaalazzeh]

introduces a generic REST Transcoding helper under google.api_core.gapic_v1.rest_transcoding.

Scope of Changes:

• rest_helpers.py: A centralized, parameters-driven generic utility transcode() that replaces the redundant code generated inside the client REST transport constructors (query parameter JSON parsing, required field default checking, body parsing, and numeric enum serialization).
• __init__.py: Registered lazy-loading for the module.
• conftest.py: Added session-scoped mock fixtures for mTLS variables to prevent local workstation client certificate leakage during python unit tests.
• test_rest_transcoding.py: A comprehensive test suite validating transcoding behavior for query parameters, nested fields, required field default injection, and numeric enums.

This is the runtime package companion to the generator PR.

[gemini-code-assist]

Code Review

This pull request introduces a new rest_transcoding module under google/api_core/gapic_v1 to handle transcoding of requests into HTTP methods, URIs, bodies, and query parameters. It also adds comprehensive unit tests for this new functionality, configures a session-scoped pytest fixture to isolate tests from workstation mTLS environments, and updates Python version configurations. The feedback suggests optimizing the conversion of protobuf messages to Python dictionaries in rest_transcoding.py by using json_format.MessageToDict instead of serializing to JSON and deserializing it back.

[hebaalazzeh]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces a new helper module rest_helpers in google.api_core.gapic_v1 containing a transcode function, which converts protobuf or proto-plus request messages into HTTP methods, URIs, JSON bodies, and query parameters. It also includes comprehensive unit tests for this function and a new session-scoped test fixture to isolate unit tests from workstation mTLS environments. There are no review comments, and I have no additional feedback to provide.