Ensure mypy passes for all packages with `--check-untyped-defs`

In PR https://github.com/googleapis/google-cloud-python/pull/16079, there were several mypy failures when `--check-untyped-defs` was added to the `mypy` command. These failures are in hybrid libraries, but more specifically the handwritten files in these libraries. The `noxfile.py` file is automatically generated for hybrid libraries which is how `--check-untyped-defs` was turned on for the hybrid libraries.

See the build log [here](https://github.com/googleapis/google-cloud-python/actions/runs/22976224095/job/66705373785).

Example failure

```
2026-03-11T22:14:04.4510210Z ~/work/google-cloud-python/google-cloud-python/packages/google-cloud-bigquery-storage ~/work/google-cloud-python/google-cloud-python
2026-03-11T22:14:04.5854802Z nox > Running session mypy-3.14
2026-03-11T22:14:04.5860181Z nox > Creating virtual environment (virtualenv) using python3.14 in .nox/mypy-3-14
2026-03-11T22:14:04.8461731Z nox > python -m pip install 'mypy<1.16.0' types-requests types-protobuf pandas-stubs
2026-03-11T22:14:10.7173130Z nox > python -m pip install '.[fastavro]'
2026-03-11T22:14:16.2114683Z nox > mypy -p google --check-untyped-defs
2026-03-11T22:14:16.3367918Z mypy.ini: [mypy-google.iam.*]: Per-module sections should only specify per-module flags (explicit_package_bases, incremental)
2026-03-11T22:15:08.0802838Z .nox/mypy-3-14/lib/python3.14/site-packages/google/cloud/bigquery_storage_v1/reader.py:150: error: Item "None" of "Any | None" has no attribute "__iter__" (not iterable) [union-attr]
2026-03-11T22:15:08.0806274Z .nox/mypy-3-14/lib/python3.14/site-packages/google/cloud/bigquery_storage_v1/reader.py:451: error: Item "None" of "Any | None" has no attribute "empty_table" [union-attr]
2026-03-11T22:15:08.0807929Z .nox/mypy-3-14/lib/python3.14/site-packages/google/cloud/bigquery_storage_v1/reader.py:553: error: Argument 1 to "next" has incompatible type "Any | None"; expected "SupportsNext[Any]" [arg-type]
2026-03-11T22:15:08.0809570Z .nox/mypy-3-14/lib/python3.14/site-packages/google/cloud/bigquery_storage_v1/reader.py:711: error: Incompatible types in assignment (expression has type "Series[Any]", target has type "list[Any]") [assignment]
2026-03-11T22:15:08.0811203Z .nox/mypy-3-14/lib/python3.14/site-packages/google/cloud/bigquery_storage_v1/reader.py:728: error: Argument 1 has incompatible type "Any | None"; expected "str | list[Any] | dict[Any, Any]" [arg-type]
2026-03-11T22:15:08.0812495Z .nox/mypy-3-14/lib/python3.14/site-packages/google/cloud/bigquery_storage_v1/reader.py:751: error: Too few arguments [call-arg]
2026-03-11T22:15:08.0813984Z .nox/mypy-3-14/lib/python3.14/site-packages/google/cloud/bigquery_storage_v1/reader.py:751: error: Argument 2 has incompatible type "Any | None"; expected "str | list[Any] | dict[Any, Any]" [arg-type]
2026-03-11T22:15:09.6739206Z .nox/mypy-3-14/lib/python3.14/site-packages/google/cloud/bigquery_storage_v1/client.py:52: error: Signature of "read_rows" incompatible with supertype "BigQueryReadClient" [override]
2026-03-11T22:15:09.6740790Z .nox/mypy-3-14/lib/python3.14/site-packages/google/cloud/bigquery_storage_v1/client.py:52: note: Superclass:
2026-03-11T22:15:09.6743230Z .nox/mypy-3-14/lib/python3.14/site-packages/google/cloud/bigquery_storage_v1/client.py:52: note: def read_rows(self, request: ReadRowsRequest | dict[Any, Any] | None = ..., *, read_stream: str | None = ..., offset: int | None = ..., retry: Retry | _MethodDefault | None = ..., timeout: float | object = ..., metadata: Sequence[tuple[str, str | bytes]] = ...) -> Iterable[ReadRowsResponse]
2026-03-11T22:15:09.6745217Z .nox/mypy-3-14/lib/python3.14/site-packages/google/cloud/bigquery_storage_v1/client.py:52: note: Subclass:
2026-03-11T22:15:09.6747140Z .nox/mypy-3-14/lib/python3.14/site-packages/google/cloud/bigquery_storage_v1/client.py:52: note: def read_rows(self, name: Any, offset: Any = ..., retry: Any = ..., timeout: Any = ..., metadata: Any = ..., retry_delay_callback: Any = ...) -> Any
2026-03-11T22:15:10.9410241Z .nox/mypy-3-14/lib/python3.14/site-packages/google/cloud/bigquery_storage_v1beta2/client.py:54: error: Signature of "read_rows" incompatible with supertype "BigQueryReadClient" [override]
2026-03-11T22:15:10.9411561Z .nox/mypy-3-14/lib/python3.14/site-packages/google/cloud/bigquery_storage_v1beta2/client.py:54: note: Superclass:
2026-03-11T22:15:10.9413591Z .nox/mypy-3-14/lib/python3.14/site-packages/google/cloud/bigquery_storage_v1beta2/client.py:54: note: def read_rows(self, request: ReadRowsRequest | dict[Any, Any] | None = ..., *, read_stream: str | None = ..., offset: int | None = ..., retry: Retry | _MethodDefault | None = ..., timeout: float | object = ..., metadata: Sequence[tuple[str, str | bytes]] = ...) -> Iterable[ReadRowsResponse]
2026-03-11T22:15:10.9415850Z .nox/mypy-3-14/lib/python3.14/site-packages/google/cloud/bigquery_storage_v1beta2/client.py:54: note: Subclass:
2026-03-11T22:15:10.9417258Z .nox/mypy-3-14/lib/python3.14/site-packages/google/cloud/bigquery_storage_v1beta2/client.py:54: note: def read_rows(self, name: Any, offset: Any = ..., retry: Any = ..., timeout: Any = ..., metadata: Any = ..., retry_delay_callback: Any = ...) -> Any
2026-03-11T22:15:12.1840694Z Found 9 errors in 3 files (checked 87 source files)
2026-03-11T22:15:12.2047664Z nox > Command mypy -p google --check-untyped-defs failed with exit code 1
2026-03-11T22:15:12.2048654Z nox > Session mypy-3.14 failed.
```

This bug tracks adding `--check-untyped-defs` to the `mypy` command for all packages, including the handwritten ones.