Stabilize side-panel Chrome acceptance

[CorsenAI]

Problem

The Linux packaged-browser gate intermittently detached from and immediately reattached to the same inactive extension page. On a slow runner, the first Runtime.evaluate on the new session could remain unanswered for 30 seconds even though the identical release tree passed on three adjacent Ubuntu jobs.

Fix

• keep the already-responsive side-panel CDP session attached
• issue the non-idempotent dismiss click exactly once with a synchronous evaluation
• observe DOM closure separately with a bounded Python-side poll
• verify persistent storage only after the notice is hidden

This changes the acceptance harness only; extension and companion release payloads are unchanged.

Validation

• 76 Python tests passed (6 Windows platform skips)
• 21 JavaScript tests passed
• real Chrome acceptance passed three consecutive local runs