[py][bidi]: add BiDi tests for userAgentClientHints

[aayushisabharwal]

🔗 Related Issues

None. This adds missing test coverage. Ruby already has the equivalent spec in rb/spec/integration/selenium/webdriver/bidi/protocol/user_agent_client_hints_spec.rb.

💥 What does this PR do?

The Python binding had no tests for userAgentClientHints.setClientHintsOverride. This adds py/test/selenium/webdriver/common/_bidi/user_agent_client_hints_tests.py with three tests:

• test_set_client_hints_override_with_contexts: sets the override for the current browsing context, navigates, and asserts that navigator.userAgentData reports the overridden platform, brands and mobile values.
• test_clear_client_hints_override: asserts the override is applied, then that passing None restores the browser's original platform.
• test_set_client_hints_override_with_user_contexts: sets the override for a new user context and asserts that a tab opened in it sees the overridden platform.

🔧 Implementation Notes

• Follows the existing user agent override tests in _bidi/emulation_tests.py (per context, per user context, reset) and mirrors the Ruby spec.
• Each override is cleared in a finally block, or discarded together with its user context, so it cannot leak into other tests that share the BiDi session.
• The tests cover Selenium's side of the command: parameter serialization, contexts / userContexts scoping, and sending null to clear. Browser conformance details such as scope precedence, high-entropy values and Sec-CH-UA* headers are left to web-platform-tests.
• Marked xfail_firefox because Firefox does not implement the userAgentClientHints module. The Ruby spec is pending on Firefox for the same reason.

Verified locally on current trunk:

• bazel test //py:test/selenium/webdriver/common/_bidi/user_agent_client_hints_tests-{chrome,edge,firefox}-bidi --cache_test_results=no: 3 passed on Chrome and Edge, 3 xfailed on Firefox.
• Negative check: with the set_client_hints_override calls temporarily removed, all 3 tests fail on Chrome (the page reports the real macOS platform instead of the override). Reverted afterwards.
• ruff check and ruff format --check are clean.

🤖 AI assistance

• No substantial AI assistance used
• AI assisted (complete below)
  • Tool(s): Claude
  • What was generated: the test file, which I reviewed, ran and verified locally
  • I reviewed all AI output and can explain the change

💡 Additional Considerations

• The tests use the generated _bidi layer because the public selenium.webdriver.common.bidi.UserAgentClientHints is currently generated without any methods. It looks like the command pattern in py/generate_bidi.py (_extract_commands) only matches params: <NamedType>, while the UA Client Hints CDDL declares its params inline (params: { ... }). If that is confirmed, a follow-up could fix the generator and add a matching bidi/user_agent_client_hints_tests.py.
• Java, .NET and JavaScript also have no tests for this command.

🔄 Types of changes

• Test additions (no change to shipped code)

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[navin772]

Could we add a test for global override where it doesn't passes contexts or user_contexts and assert the expected behaviour