fix(chatd): publish streaming message_part events during compaction (#22410)

## Problem

Context compaction in chatd persisted durable messages for the
`chat_summarized` tool call and result via `publishMessage`, but never
published `message_part` streaming events via `publishMessagePart`. This
meant connected clients had no streaming representation of the
compaction.

The client's `streamState` (built entirely from `message_part` events in
`streamState.ts`) never saw the compaction tool call, so:

- No **"Summarizing..."** running state was shown to the user during
summary generation (which can take up to 90s).
- The durable `message` events arrived after or interleaved with the
`status: waiting` event, causing the tool to appear as "Summarized" with
the chat appearing to just stop.

## Fix

### 1. `CompactionOptions.OnStart` callback (chatloop)

Added an `OnStart` callback to `CompactionOptions`, called in
`maybeCompact` right before `generateCompactionSummary` (the slow LLM
call). This gives `chatd` a hook to publish the tool-call `message_part`
immediately when compaction begins.

### 2. Tool-result streaming part (chatd)

`persistChatContextSummary` now publishes a tool-result `message_part`
before the durable `message` events, so clients transition from
"Summarizing..." to "Summarized" before the status change arrives.

### Event ordering is now:
1. `message_part` (tool call via `OnStart`) — client shows
"Summarizing..."
2. LLM generates summary (up to 90s)
3. `message_part` (tool result) — client shows "Summarized" in stream
state
4. `message` (assistant) — durable message persisted, stream state
resets
5. `message` (tool) — durable tool result persisted
6. `status: waiting` — chat transitions to idle

## Tests

- **`OnStartFiresBeforePersist`**: Verifies callback ordering is
`on_start` → `generate` → `persist`.
- **`OnStartNotCalledBelowThreshold`**: Verifies `OnStart` is not called
when context usage is below the compaction threshold.

coderd/chatd/chatloop/compaction.go

@@ -30,6 +30,7 @@ type CompactionOptions struct {
 	SystemSummaryPrefix string
 	Timeout             time.Duration
 	Persist             func(context.Context, CompactionResult) error
+	OnStart             func()
 	OnError             func(error)
 }
 
@@ -134,6 +135,10 @@ func maybeCompact(
 		return nil
 	}
 
+	if config.OnStart != nil {
+		config.OnStart()
+	}
+
 	summary, err := generateCompactionSummary(
 		ctx,
 		runOpts.Model,