[gapic-generator] Port active PRs and logic from legacy gapic-generator-python repository

[ohmayr]

This issue tracks the identification and manual porting of high-value PRs from the legacy repository (https://github.com/googleapis/gapic-generator-python) into packages/gapic-generator-python/.

Porting Plan
To preserve authorship and history while adjusting for the new mono-repo directory structure, use the following Path-Aware Patch strategy:

1. Identify the Base: Use the migration snapshot commit (<MIGRATION_SHA>) as the starting point.

2. Export from Legacy: In the old repo:

git format-patch <MIGRATION_SHA>..feature-branch --stdout > feature.patch

3. Import to Mono-repo: In google-cloud-python:

git am --directory=packages/gapic-generator-python/ < feature.patch

4. Verify: Run nox -s unit within the package directory to ensure the logic survived the move.