fix(assistant): trailing-edge flush for throttled message updates

[juacker]

Problem

When an agent makes parallel tool calls, both run, but the chat shows only the first one. The second appears only when the next batch of tool calls or the end of the turn arrives.

Root cause: the 200 ms throttle on assistant_message_updated (Claude, Codex exec, Codex app-server and OpenCode paths in local_agent.rs) was leading-edge only. An update suppressed inside the window was never re-sent. The database row always had every tool call; only the live UI event was lost. Codex emits parallel tool calls 3–53 ms apart, and Claude's adopt_complete_assistant_message persists several tool calls in a loop, so both paths hit it.

• The frontend renders tool rows only from message content, so it needs no change.
• This predates perf(assistant): run-local history and token-based compaction #219: the throttle dates from May.

Fix

• One throttle helper. UpdateThrottle<T> replaces three copy-pasted throttles. It emits immediately, or holds the latest update and releases it when the window ends.
• Trailing emit. Each read loop adds a timer branch to its select!, so a held update reaches the UI within about 200 ms even while a tool runs for minutes. The rate stays capped at 5 updates per second.
• Delta ordering. A held update is always sent before any AssistantDelta (emit_assistant_delta / delta_events). Otherwise a late snapshot would wipe text that had already streamed, because updateMessageContent resets the delta accumulator.
• Error exits. Read, parse and handler errors, child.wait(), and the finalize failures in cancel, loop-end, rotate and split all send the held update first.
• Completion, rotate and split. The held update is dropped, because AssistantMessageCompleted already carries the full content. Nothing is emitted after completion or under a stale message id.

Tests

• update_throttle*: covers holding, latest-wins, window restart, and two tool calls 3 ms apart where the last emit contains both.
• sleep_until_due: checked with paused tokio time.
• delta_events_emit_held_update_before_delta: checks the ordering.
• Changing offer back to the old drop-on-hold behaviour fails 4 of the 5 throttle tests.

No loop-level test exists, because the repo has no Tauri mock app.

Review

• Round 1 returned needs_work: a stale snapshot could erase streamed text, and two minor findings. All three were fixed in f37f05981.
• Round 2 returned production_quality.

Manual check

With a Codex- or Claude-backed agent, ask for two parallel tool calls, such as two workspace_assignTask calls. Both should appear in the chat immediately.