fix: support async predicates in page.expect_request/expect_response

[Skn0tt]

Closes #1545.

Async predicates passed to page.expect_request / page.expect_response were called but never awaited, so the returned coroutine evaluated truthy and the first matching event resolved the wait (with a RuntimeWarning: coroutine '...' was never awaited).

• Waiter now detects coroutine results from predicates and awaits them in a tracked task; exceptions from predicates reject the waiter instead of being silently swallowed by pyee.
• The async API's predicate parameter type is widened to Callable[[T], Union[bool, Awaitable[bool]]]. The sync API keeps Callable[[T], bool] — codegen was extended with a SYNC_API flag that strips the Awaitable arm from both signatures and docstrings for the sync generator.
• Two tests added in tests/async/test_page.py covering the success path and exception propagation.

[dgozman]

Is this line a separate bugfix? I don't see where we would reject after a sync predicate throws before this PR. Perhaps add a test for this as well?

[Skn0tt]

Good catch — yes, this is a coupled bugfix. Previously sync predicates that threw were silently swallowed by pyee's listener machinery, leaving the expect_* call to hang until timeout. Pushed a test (test_expect_response_should_reject_when_sync_predicate_throws) covering it.

[Skn0tt]

Cross-checked against JS waiter.ts's waitForEvent (packages/playwright-core/src/client/waiter.ts:113-124) — JS already awaits predicates and rejects on either sync or async predicate exceptions (try/catch wraps await predicate(eventArg)). This PR brings Python to parity.