Latest mypy checks fail for several generated packages

[parthea]

PR #2530 updated the mypy version to the latest.

This change is blocking googleapis/google-cloud-python#16063 where there are several mypy failures.

See https://github.com/googleapis/google-cloud-python/actions/runs/22913070856/job/66490664690?pr=16063

As an example

checking changes with 'git diff --quiet origin/main... packages/google-cloud-artifact-registry/'
change detected in packages/google-cloud-artifact-registry/
running test in packages/google-cloud-artifact-registry/
~/work/google-cloud-python/google-cloud-python/packages/google-cloud-artifact-registry ~/work/google-cloud-python/google-cloud-python
nox > Running session mypy-3.14
nox > Creating virtual environment (virtualenv) using python3.14 in .nox/mypy-3-14
nox > python -m pip install mypy types-requests types-protobuf
nox > python -m pip install .
nox > mypy -p google --check-untyped-defs
mypy.ini: [mypy-google.iam.*]: Per-module sections should only specify per-module flags (explicit_package_bases, incremental)
.nox/mypy-3-14/lib/python3.14/site-packages/google/cloud/artifactregistry_v1beta2/services/artifact_registry/client.py:3335: error: Item "dict[Any, Any]" of "Any | dict[Any, Any]" has no attribute "resource"  [union-attr]
.nox/mypy-3-14/lib/python3.14/site-packages/google/cloud/artifactregistry_v1beta2/services/artifact_registry/client.py:3450: error: Item "dict[Any, Any]" of "Any | dict[Any, Any]" has no attribute "resource"  [union-attr]
.nox/mypy-3-14/lib/python3.14/site-packages/google/cloud/artifactregistry_v1beta2/services/artifact_registry/client.py:3537: error: Item "dict[Any, Any]" of "Any | dict[Any, Any]" has no attribute "resource"  [union-attr]
.nox/mypy-3-14/lib/python3.14/site-packages/google/cloud/artifactregistry_v1beta2/services/artifact_registry/async_client.py:2832: error: Item "dict[Any, Any]" of "Any | dict[Any, Any]" has no attribute "resource"  [union-attr]
.nox/mypy-3-14/lib/python3.14/site-packages/google/cloud/artifactregistry_v1beta2/services/artifact_registry/async_client.py:2948: error: Item "dict[Any, Any]" of "Any | dict[Any, Any]" has no attribute "resource"  [union-attr]
.nox/mypy-3-14/lib/python3.14/site-packages/google/cloud/artifactregistry_v1beta2/services/artifact_registry/async_client.py:3036: error: Item "dict[Any, Any]" of "Any | dict[Any, Any]" has no attribute "resource"  [union-attr]
.nox/mypy-3-14/lib/python3.14/site-packages/google/cloud/artifactregistry_v1/services/artifact_registry/client.py:5594: error: Item "dict[Any, Any]" of "Any | dict[Any, Any]" has no attribute "resource"  [union-attr]
.nox/mypy-3-14/lib/python3.14/site-packages/google/cloud/artifactregistry_v1/services/artifact_registry/client.py:5709: error: Item "dict[Any, Any]" of "Any | dict[Any, Any]" has no attribute "resource"  [union-attr]
.nox/mypy-3-14/lib/python3.14/site-packages/google/cloud/artifactregistry_v1/services/artifact_registry/client.py:5796: error: Item "dict[Any, Any]" of "Any | dict[Any, Any]" has no attribute "resource"  [union-attr]
.nox/mypy-3-14/lib/python3.14/site-packages/google/cloud/artifactregistry_v1/services/artifact_registry/async_client.py:4959: error: Item "dict[Any, Any]" of "Any | dict[Any, Any]" has no attribute "resource"  [union-attr]
.nox/mypy-3-14/lib/python3.14/site-packages/google/cloud/artifactregistry_v1/services/artifact_registry/async_client.py:5075: error: Item "dict[Any, Any]" of "Any | dict[Any, Any]" has no attribute "resource"  [union-attr]
.nox/mypy-3-14/lib/python3.14/site-packages/google/cloud/artifactregistry_v1/services/artifact_registry/async_client.py:5163: error: Item "dict[Any, Any]" of "Any | dict[Any, Any]" has no attribute "resource"  [union-attr]
Found 12 errors in 4 files (checked 58 source files)

As a temporary fix, we should revert mypy back to 1.16.0 to unblock other features

[parthea]

See googleapis/google-cloud-python#16063 (comment)