Fix ambient session cleanup on pane close

[zachbai]

Summary

• Unregister ambient session mappings for every terminal view in a pane stack when a pane detaches, and re-register them when it reattaches.
• Replace stale ambient task mappings when the same task is registered for a newer terminal view.
• Ignore hidden panes when looking for an already-open ambient agent conversation.

Tests

• CARGO_BUILD_JOBS=1 cargo nextest run --manifest-path /workspace/warp/Cargo.toml -p warp -E 'test(ambient_session_registration_replaces_stale_terminal_for_same_task) | test(ambient_session_unregister_keeps_task_open_until_last_terminal_is_removed) | test(test_resolve_open_action_reopens_ambient_session_after_terminal_unregister)'

Co-Authored-By: Oz oz-agent@warp.dev

Conversation: https://staging.warp.dev/conversation/cea617fb-5ea0-4e0b-8f27-6078accad759
Run: https://oz.staging.warp.dev/runs/019e0919-5caa-7ec5-acd6-160481714093
This PR was generated with Oz.

[zachbai]

• Fix ambient session cleanup on pane close #10510 👈 (View in Graphite)
• master

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