## 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
9c099e94a2