fix: prevent CDP connect hang on browsers with discarded/frozen tabs

[ChunHao-dev]

Summary

Fix CDP connect hanging indefinitely on browsers with discarded/frozen tabs (Chrome Memory Saver).

Fixes #1036

Changes

• Add 3-second timeout to enable_domains() call during initial CDP connection in discover_and_attach_targets()
• If the first tab fails to respond (likely discarded/frozen with no renderer process), fall back to creating a new about:blank tab
• All existing tabs remain visible in tab list (attached but not enabled); they get enabled lazily on tab switch

Test plan

• cargo fmt --check ✅
• cargo clippy ✅
• cargo test — 565 passed, 0 failed ✅
• Manual test: chrome://discards → Urgent Discard multiple tabs → agent-browser --cdp 9222 get title
  • Before: hangs indefinitely
  • After: completes in ~3s with fallback to new tab

Design decision

Fallback to a new tab after the first timeout rather than trying all tabs because:

1. Target.getTargets order is not sorted by activity — there's no way to know which tab is more likely to be active, so trying them sequentially doesn't improve the odds
2. Each timeout costs 3 seconds — with many discarded tabs, sequential retries would add up (e.g. 10 discarded tabs = 30s wait)
3. Creating a new tab is instant and guaranteed to work — it always has a live renderer

A future improvement could use the /json/list HTTP endpoint (which returns tabs sorted by most recently active) to pick a better first candidate.

[vercel]

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