## Why
Switching back to a running thread can lose the active turn's state after its `TurnStarted` notification falls out of the bounded replay buffer, breaking steering.
## What changed
Retain the evicted active turn and include it in replay snapshots when it is missing from the stored turns. Clear the retained turn when turns are replaced, the active turn completes, or the thread closes.
## Testing
Add a regression test that evicts the turn-start notification, restores the thread, and submits two steering messages while preserving a queued follow-up. A snapshot verifies the steering messages and queued input remain visible.
GitOrigin-RevId: a4302a2e0990b907207c229361cbcec68ebbbdc4
## Why
The per-thread replay buffer limited its event count, but streamed agent-message
deltas could still retain an unbounded amount of text while a thread was
inactive.
## What changed
- Coalesce adjacent deltas for the same thread, turn, and item into chunks of up
to 4 KiB.
- Limit buffered agent-message deltas to 256 KiB, discard a single delta larger
than that limit, and evict the oldest events when either buffer limit is
exceeded.
- Keep pending input and approval state in sync when eviction removes the
corresponding request, including clearing side-thread status indicators.
## Testing
Added coverage for delta coalescing, byte-based eviction, oversized deltas,
replaying only retained text, and side-thread status updates after eviction.
GitOrigin-RevId: 88d85353a124467d467822d2fd9854312b68d547
Tamir Duberstein