Address testing gap with `_pypi_packages.j2`

The latest release https://github.com/googleapis/gapic-generator-python/releases/tag/v1.30.10 caused failures downstream in https://github.com/googleapis/google-cloud-python/pull/16079 because of a testing gap with `_pypi_packages.j2`.

See the failure in https://github.com/googleapis/google-cloud-python/actions/runs/22982079612/job/66723950887?pr=16079

```
2026-03-12T01:37:57.4367037Z ERROR: Cannot install google-api-core[grpc]==2.21.0, google-cloud-asset and google-cloud-asset==4.2.0 because these package versions have conflicting dependencies.
2026-03-12T01:37:57.4367632Z
2026-03-12T01:37:57.4367732Z The conflict is caused by:
2026-03-12T01:37:57.4368078Z google-cloud-asset 4.2.0 depends on google-api-core<3.0.0 and >=2.11.0
2026-03-12T01:37:57.4369340Z google-api-core[grpc] 2.21.0 depends on google-api-core 2.21.0 (from https://files.pythonhosted.org/packages/6a/ef/79fa8388c95edbd8fe36c763259dade36e5cb562dcf3e85c0e32070dc9b0/google_api_core-2.21.0-py3-none-any.whl (from https://pypi.org/simple/google-api-core/) (requires-python:>=3.7))
2026-03-12T01:37:57.4370871Z google-cloud-access-context-manager 0.1.2 depends on google-api-core<2.0.0dev and >=1.14.0
2026-03-12T01:37:57.4371396Z The user requested (constraint) google-api-core==2.21.0
2026-03-12T01:37:57.4371632Z
2026-03-12T01:37:57.4371951Z Additionally, some packages in these conflicts have no matching distributions available for your environment:
2026-03-12T01:37:57.4372442Z google-api-core
2026-03-12T01:37:57.4372569Z
2026-03-12T01:37:57.4372673Z To fix this you could try to:
2026-03-12T01:37:57.4372967Z 1. loosen the range of package versions you've specified
2026-03-12T01:37:57.4373415Z 2. remove package versions to allow pip to attempt to solve the dependency conflict
2026-03-12T01:37:57.4373731Z
2026-03-12T01:37:57.4374173Z ERROR: ResolutionImpossible: for help visit https://pip.pypa.io/en/latest/topics/dependency-resolution/#dealing-with-dependency-conflicts
2026-03-12T01:37:57.4374706Z
2026-03-12T01:37:57.4374878Z nox > Session unit-3.9(protobuf_implementation='python') failed.
```

https://github.com/googleapis/gapic-generator-python/pull/2582 includes a fix for this specific issue but we still need to figure out how to address the testing gap