Remove runtime pkg_resources dependency in default and server_ingester

[0xmagnus91]

Motivation for features / changes

Fixes #7003.

pkg_resources is removed in newer setuptools, which can break TensorBoard at import/runtime. This change removes runtime reliance on pkg_resources in the two affected code paths.

Technical description of changes

• Replaced dynamic plugin discovery in tensorboard/default.py:
  • from pkg_resources.iter_entry_points(...)
  • to importlib.metadata.entry_points(...) with compatibility handling for different Python return shapes.
• Replaced version parsing in tensorboard/data/server_ingester.py:
  • from pkg_resources.parse_version(...)
  • to packaging.version.parse(...).
• Updated related tests:
  • tensorboard/default_test.py now patches _iter_entry_points and uses load()-style fake entry points.
  • tensorboard/version_test.py now validates PEP 440 behavior using packaging.version.
• Updated Bazel deps to use expect_packaging_installed where packaging is now required.

Detailed steps to verify changes work correctly (as executed by you)

Executed locally in an isolated venv:

• python -m py_compile tensorboard/default.py tensorboard/default_test.py tensorboard/data/server_ingester.py tensorboard/version_test.py
• PYTHONPATH=. python tensorboard/version_test.py (passes)

Attempted but environment-limited locally:

• bazel test //tensorboard:version_test //tensorboard:default_test //tensorboard/data:server_ingester_test (bazel not available in local shell)
• Direct execution of default_test.py and server_ingester_test.py without Bazel-generated artifacts hit local environment/import constraints.

Alternate designs / implementations considered (or N/A)

• N/A

[0xmagnus91]

Added local validation results while workflow runs are pending maintainer approval for fork PR CI.

Local command executed:
bazel test //tensorboard:version_test //tensorboard:default_test //tensorboard/data:server_ingester_test

Result: all 3 targets passed locally.

Could a maintainer approve the pending workflow runs so required checks can execute on this PR? Thanks.