[Pavel Feldman (pavelfeldman)]

@pytest.mark.parametrize("browser_type_launch_args", [{...}], indirect=True) was silently ignored by the built-in fixture. It now merges request.param into the launch options, on top of the command line options (--headed, --browser-channel, --slowmo). The fixture is session scoped, so pytest relaunches the browser for each distinct value and groups the tests accordingly. A value that is not a mapping raises pytest.UsageError naming the fixture instead of failing inside dict.update.

Tests: two parametrized user agents plus an unparametrized test in one run, and the error for a non-mapping value (sync + async).

Docs follow-up (microsoft/playwright docs/src/test-runners-python.md): document the parametrization and that each distinct value launches its own browser.

Fixes #205

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