feat: adds support for Python runtime 3.14

[chalmerlowe]

This PR updates the python-bigquery library to add support for Python 3.14.

The following changes have been made:

• Added the trove classifier for Python 3.14 in pyproject.toml.
• Added Python 3.14 to the test matrix in the GitHub Actions workflow (.github/workflows/unittest.yml).
• Created a new constraints file testing/constraints-3.14.txt to include a minimum version for grpcio (>= 1.75.1) required by Python 3.14.
• Updated CONTRIBUTING.rst to list Python 3.14 as a supported version.
• Added Samples - Python 3.14 to the required status checks in .github/sync-repo-settings.yaml.
• Included "3.14" in the unit_test_python_versions list in owlbot.py.
• Added the grpcio >= 1.75.1 constraint for python_version >= '3.14' in the optional dependencies
  (bqstorage, pandas) in pyproject.toml.
• Updated UNIT_TEST_PYTHON_VERSIONS in noxfile.py to include "3.14".

These changes ensure that the library is tested against Python 3.14 and declares support for it, while also handling the new grpcio dependency requirement.

[vchudnov-g]

Looking at the GitHub presubmits, I see the unit tests for 3.14 ran, but the sample tests for 3.14 did not . Neither did Kokoro snippets and Kokoro system, for 3.14. It looks from our guide that at least the samples test should be configured to run. And it would be great to get the Kokoro test to run right away as well, though tbh i don't know whether those depend on additional configs.

[vchudnov-g]

Could you file an issue to track this, and mention it in the TODO. Something like:
TODO(http://....): Re-enable tests on Python 3.14 once....

[chalmerlowe]

Github issues automatically become buganizer issues for this repo. There is no need for me to file an issue.
I will add the buganizer number, per request.

[vchudnov-g]

Why is 3.10 missing (even before this PR)? Either we test on all supported versions, or we sample fewer (like the lowest and highest), don't you think?

[chalmerlowe]

Updated.

[chalmerlowe]

@vchudnov-g

DONE:

Note: there is an internal CL to add Samples for 3.14 as a check in presubmit: https://critique.corp.google.com/cl/821648116

[vchudnov-g]

Thanks for doing this! Just one non-blocking cosmetic comment.

[vchudnov-g]

Let's avoid references to internal issues where we can.

Suggested change

	# TODO (https://b.corp.google.com/issues/450370502) 3.14 is not yet supported by pyarrow. See
	# https://github.com/googleapis/google-cloud-python/issues/14686
	# https://github.com/apache/arrow/issues/47438
	# Reinstate running tests with 3.14 once this bug is fixed
	# TODO (https://github.com/googleapis/google-cloud-python/issues/14686) Reinstate running
	# tests with 3.14 once Python 3.14 is supported by pyarrow.
	# See also https://github.com/apache/arrow/issues/47438