fix(mcp): report a dialog opened during navigation instead of timing out

[AbNoZ&v02 (Abnoz01)]

Summary

• A page that opens a dialog while loading never reaches domcontentloaded, so browser_navigate blocked for the full navigation timeout (60s by default) and then failed with a TimeoutError, only exposing the modal state afterwards.
• Race page.goto against a newly appearing modal state and return as soon as the dialog shows up, so the response reports it and browser_handle_dialog can continue, matching how actions already behave. Add a test.

Fixes #42817

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