fix: avoid stale injected script installs after navigation

[coopjyoung]

Fixes #41773

Problem

Frame.extendInjectedScript() could continue after a navigation if the main world context promise was still pending.

A stale injection request from the previous document could resolve using the new document's context, causing duplicate injected scripts.

Solution

Capture the current document before awaiting the main world context. If the frame navigated while waiting, the stale operation exits without injecting.

Testing

Added a regression test that deterministically reproduces the navigation race.

Verified:

• regression test fails before the fix
• regression test passes after the fix
• existing selector-generator tests continue passing

[coopjyoung]

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