fix: validate mutually-exclusive body options under python -O

[Suraj-Self (suraj-self)]

Fixes #43281

Summary

Replaces the assert-based mutual-exclusion guard in
APIRequestContext._inner_fetch() with an explicit conditional that raises
Error. assert statements are stripped under python -O, which previously
allowed conflicting data / form / multipart arguments to bypass
validation silently.

Also switches the check to presence semantics (is not None) so that empty
values such as data="" are counted as provided, consistent with how the
value is used later in the method.

Tests

• Updated the existing test_should_reject_empty_data_with_form (sync + async)
  to expect Error instead of AssertionError.
• Added test_should_reject_multiple_body_options_under_python_optimization
  (sync + async), which spawns a child interpreter with -O and verifies the
  guard still raises. Confirmed the new test fails on unpatched main and
  passes with the fix.

[Pavel Feldman (pavelfeldman)]

Not sure why assert error is gone, also we don't need a test for such a minor use case.

[Suraj-Self (suraj-self)]

Not sure why assert error is gone, also we don't need a test for such a minor use case.

Thanks for reviewing. The explicit guard is needed because assert statements are removed under python -O. I changed the exception to Playwright's Error to match API validation conventions, but I agree that preserving AssertionError keeps this fix narrower. I'll simplify the change and remove the additional optimization-specific tests.

[Pavel Feldman (pavelfeldman)]

We might need a larger change that addressees all the user-facing asserts.

[Suraj-Self (suraj-self)]

We might need a larger change that addressees all the user-facing asserts.

Agreed. This issue exposed one case where user-facing validation is disabled under python -O. A broader audit of user-facing asserts makes sense, and I'd be happy to help work on that as well.

[Pavel Feldman (pavelfeldman)]

I'll close this one since it is just a one-off