fix: keep multi-browser parametrization when the page fixture is overridden

[Sebastian Cao (cycsmail)]

Summary

• When a user overrides the page fixture (e.g. def page(page): yield page), some pytest versions drop browser_name from the fixture closure, so --browser chromium --browser firefox silently runs the test against only one browser instead of parametrizing it.
• In pytest_generate_tests, if browser_name is absent but a browser-backed fixture (page, context, new_context, browser, browser_type) is requested, re-add browser_name to metafunc.fixturenames before parametrizing. It's a no-op when browser_name is already present (pytest 9) or when no browser fixture is used, so non-browser tests are untouched.
• Applied to both the sync and asyncio plugins, with a regression test in each that collects test_a[chromium] and test_a[firefox] from an overridden page fixture.

Reverting the plugin change makes the new tests fail on pytest 8.3.5 (only test_a is collected), confirming they guard the regression. On pytest 9 the closure already keeps browser_name, so the tests pass there regardless.

Fixes #172

[Sebastian Cao (cycsmail)]

@microsoft-github-policy-service agree