fix: replace deprecated pkg_resources namespace declaration

[Tranquil-Flow]

Summary

Replace the deprecated pkg_resources.declare_namespace() namespace declaration in lark_oapi/ws/pb/google/__init__.py with the existing pkgutil.extend_path() fallback path.

Why

Importing this module currently emits:

UserWarning: pkg_resources is deprecated as an API ... Refrain from using this package or pin to Setuptools<81.

In newer setuptools environments, this can become a hard import failure when pkg_resources.declare_namespace is unavailable.

Notes

This is the same minimal approach as PR #124, but that PR is currently blocked by CLA because its commit author email is not associated with a GitHub user. Opening this replacement so the fix has an unblocked path to merge.

Fixes #108.
Refs #115.
Refs #124.

Verification

Ran a minimal import proof with warnings treated as errors:

python -W error::UserWarning - <<'PY'
import importlib.util
from pathlib import Path
p = Path('lark_oapi/ws/pb/google/__init__.py').resolve()
spec = importlib.util.spec_from_file_location(
    'lark_oapi.ws.pb.google',
    p,
    submodule_search_locations=[str(p.parent)],
)
mod = importlib.util.module_from_spec(spec)
spec.loader.exec_module(mod)
assert mod.__path__
print('ok: lark_oapi.ws.pb.google imports without pkg_resources warning')
PY

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