fix(ui): isolate event stream state by run id

[Anuppaul]

Summary

Fix useEventStream retaining the previous run's events, status, and error when React reuses the hook instance for a different runId.

• track the run id that owns the current stream state
• reset to an empty connecting state only when the run id changes
• preserve accumulated history when only follow changes for the same run
• ignore late event/status callbacks from a previous run
• add pure helper coverage for same-run retention and cross-run reset

Closes #1325

Verification

The regression coverage is in src/ui/src/hooks/use-event-stream.helpers.test.ts and keeps the existing replay-deduplication cases unchanged.

This contribution is AI-assisted. The new regression test specifically pins the state transition that did not exist on main (new run id -> empty events/status/error). I could not run the local Bun gate from this execution environment because outbound repository cloning is unavailable, so I am relying on the repository CI check:all gates for executable verification rather than claiming a local pass.