Fix mypy presubmit failure for the generated libraries.

[ohmayr]

mypy presubmit is failing on #15532. On a first glance, the issue seems to be related to types not being ignored for bigquery storage as per the following log:

using --check-untyped-defs  [annotation-unchecked]
.nox/mypy-3-14/lib/python3.14/site-packages/google/cloud/bigquery_storage_v1beta2/__init__.py:98: note: By default the bodies of untyped functions are not checked, consider using --check-untyped-defs  [annotation-unchecked]
Found 2 errors in 1 file (checked 87 source files)
nox > Command mypy -p google failed with exit code 1
nox > Session mypy-3.14 failed.

Investigate and fix the issue