[warp-agent-staging]

Description

Keep the agent status bar's history subscription constant as exchanges change. WarpUI owns subscriptions for the view's lifetime, so replacing a boxed exchange model does not remove its callback.

Handle output events in the existing lifetime subscription, using the current exchange IDs. Older exchanges no longer reprocess the current output or close its summarization dialog. Same-conversation invalidation and global usage notifications remain supported; other AI block subscribers are unchanged.

This addresses one source of growing main-thread work during long agent conversations, not the separate Markdown parsing and stream-draining hotspots.

Linked Issue

No separate issue for this narrowly scoped fix.

• The linked issue is labeled ready-to-spec or ready-to-implement. — Not applicable.
• Where appropriate, screenshots or a short video of the implementation are included below (especially for user-visible or UI changes). — The requester chose targeted Rust tests only.

Testing

• The stale-exchange regression failed against the original callback behavior and passes with the fix. The test observes dialog events after switching exchanges and confirms the current exchange still processes output across terminal surfaces.
• CARGO_BUILD_JOBS=2 CARGO_PROFILE_DEV_DEBUG=0 cargo nextest run -p warp --lib -E 'test(ai::blocklist::block::status_bar::tests)': 16 passed.
• CARGO_BUILD_JOBS=2 CARGO_PROFILE_DEV_DEBUG=0 cargo clippy -p warp --all-targets --tests -- -D warnings: passed.
• ./script/format: passed, run last.
• I have manually tested my changes locally with ./script/run — Skipped per the requester's tests-only choice; no macOS beachball reproduction is claimed.

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode

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