Stabilize adaptive browser exploration tests

[chubes4]

Summary

• replace fixed-delay cancellation with a fixture-observed lifecycle event
• reuse one Chromium process while retaining isolated browser contexts per scenario
• replace the redundant second mobile defect exploration with deterministic environment-fingerprint coverage
• keep all existing behavioral assertions while reducing browser startup and timing-race exposure

Fixes #2421

Verification

• formerly flaky rediscovery and cancellation cases passed 5 consecutive focused runs
• full tests/browser-adaptive-exploration.test.ts passed 3 consecutive runs
• npm run build
• npm run check (343 smoke commands passed, including 330 discovered files and 9 serial files)

AI Assistance

OpenAI GPT-5.6-sol via OpenCode analyzed two distinct release-gate flakes, replaced wall-clock synchronization with an observed fixture event, consolidated browser process setup, removed redundant browser execution, and ran the listed verification. Chris Huber directed and reviewed the cleanup.