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

[parthea]

In PR #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.

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.