fix(ai): bound Summarizer availability probes

[StefanSa]

Some Chromium-based browsers expose the Summarizer API while
Summarizer.availability() can remain pending indefinitely.

Because the AI signal participates in the aggregate fingerprint collection,
one unresolved availability probe can block the complete fingerprint forever.

This was reproduced with Microsoft Edge 154, where the API is exposed but
availability() did not settle. The same general failure class previously
affected Opera GX (#67).

This change bounds both Summarizer availability probes to 2.5 seconds and
falls back to the existing ERROR sentinel when a probe does not settle.

Normal resolved values and rejection handling remain unchanged, as does the
existing Opera exclusion.

Tests cover absent API, normal resolution/rejection, either or both probes
remaining pending, bounded completion, and existing Opera behavior.

Validation:

• Edge 154: PASS after fix
• Chrome 154: PASS
• Firefox 157: PASS
• Production challenge flow: one successful submit, valid challenge cookie,
  and no challenge loop

[buixor]

I can confirm the issue on google-chrome 154.0.8037.57-1 when started on a new user profile. Summarizer.availability() never settles on a Chrome profile that hasn't already resolved its on-device model state.

[l4rm4nd]

Any updates on this?