Fix Dataflow ValueProvider serialization

[bvolpato]

Fixes #39499.

Python Dataflow client migration changed ValueProvider serialization to str(v). Unset runtime providers were consequently sent in sdk_pipeline_options as their object representation, which Flex Template workers treated as the runtime value.

This restores previous JSON behavior:

• accessible providers serialize their resolved value
• inaccessible providers serialize as null
• focused coverage checks both static and unresolved runtime providers
• CHANGES.md documents the 2.76.0 bug fix

Reproduction

Before the fix, the new test fails with:

AssertionError: 'RuntimeValueProvider(option: at_vp_arg1, type: str, default_value: None)' is not None

The same test passes after restoring value-aware serialization.

Testing

• uvx tox -e py311-cloud -- apache_beam/runners/dataflow/internal/apiclient_test.py -k test_value_provider_options_serialization
• target/.tox/py311-cloud/bin/pytest -q apache_beam/runners/dataflow/internal/apiclient_test.py apache_beam/internal/gcp/json_value_test.py (78 passed, 1 skipped)
• uvx pre-commit run --files CHANGES.md sdks/python/apache_beam/runners/dataflow/internal/apiclient.py sdks/python/apache_beam/runners/dataflow/internal/apiclient_test.py

---

• Linked relevant issue.
• Updated CHANGES.md.
• Small contribution; ICLA not applicable.

[gemini-code-assist]

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[github-actions]

Assigning reviewers:

R: @shunping for label python.

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[bvolpato]

retest this please