fix(agents): remove optimistic message rendering and fix auto-promote delivery (#22588)

## Problem

Two bugs in the agents chat flow:

1. **Optimistic rendering glitch**: When sending a message while the
agent is busy, a fake message with a negative ID appears in the
timeline, then gets rolled back to the queued state. This causes a
jarring flash.

2. **Auto-promoted messages not appearing**: When the server
auto-promotes a queued message after finishing a task, the promoted user
message doesn't show up in the timeline until the LLM finishes its
response.

## Root Causes

**Bug 1**: The optimistic rendering system injected placeholder messages
with `id: -Date.now()` into the store. When the server responded with
`queued: true`, the optimistic message was rolled back — but the user
had already seen it flash in the timeline.

**Bug 2**: In `processChat`'s deferred cleanup, the auto-promoted
message was published via `publishEvent()`, which only delivers to local
in-process stream subscribers. The SSE subscriber goroutine only
forwards `message_part` events from the local channel — it ignores
`message` events. Durable events reach the SSE client via pubsub → DB
read, but `publishEvent` doesn't trigger a pubsub notification. The
explicit `PromoteQueued` endpoint correctly used `publishMessage()`
(which does both), but the auto-promote path did not.

## Changes

### Frontend (`site/`)
- **AgentDetail.tsx**: Remove optimistic message injection from send and
edit flows. Instead, use the `CreateChatMessageResponse.message` from
the POST response to insert the real server message into the store
immediately.
- **ChatContext.ts**: Remove the negative-ID cleanup logic from
`upsertDurableMessage` that stripped optimistic placeholders when real
messages arrived.
- **chatStore.test.ts**: Remove 2 tests for negative-ID optimistic
message behavior.

### Backend (`coderd/chatd/`)
- **chatd.go**: In `processChat` cleanup, replace `publishEvent()` with
`publishMessage()` for auto-promoted messages. This ensures the pubsub
notification (`AfterMessageID`) is sent, so SSE subscribers read the new
message from the DB immediately.

coderd/chatd/chatd.go

@@ -1766,11 +1766,7 @@ func (p *Server) processChat(ctx context.Context, chat database.Chat) {
 					} else {
 						status = database.ChatStatusPending
 
-						sdkMsg := db2sdk.ChatMessage(msg)
-						p.publishEvent(chat.ID, codersdk.ChatStreamEvent{
-							Type:    codersdk.ChatStreamEventTypeMessage,
-							Message: &sdkMsg,
-						})
+						p.publishMessage(chat.ID, msg)
 
 						remaining, qErr := tx.GetChatQueuedMessages(cleanupCtx, chat.ID)
 						if qErr == nil {