fix(coderd): deflake TestChatMessageWithFiles/FileCapExceeded (#28091)

Fixes the flake tracked in
[CODAGT-926](https://linear.app/codercom/issue/CODAGT-926/flake-testchatmessagewithfilesfilecapexceeded).

## Problem

`TestChatMessageWithFiles/FileCapExceeded` asserted the rollback of a
rejected over-cap send by comparing message counts taken before and
after the send. `CreateChat` starts assistant generation asynchronously,
so the assistant reply can be persisted between the two reads, making
the count check fail even though the rejected message was correctly
rolled back ("should have 1 item(s), but has 2").

## Fix

Replace the count comparison with a semantic assertion that the rejected
`one too many` message was not persisted, hardened through Codex review
rounds:

- Scan message history for the rejected marker instead of comparing
counts.
- Also scan `QueuedMessages`: a busy chat queues the send before
file-link validation, so a rollback regression could leave the rejected
message queued rather than in history.
- Close the queue-promotion race: `getChatMessages` reads history and
the queue in two separate database reads, so the assertion first waits
for the queue to observe empty; a promoted message must then appear in a
fresh history read.

## Verification

- Deterministic repro of the exact CI failure signature: waiting for the
async assistant reply before the old count assertion reproduced `should
have 1 item(s), but has 2` every run.
- The new assertion passes under that same forced condition.
- Assertion liveness (all temporary red checks reverted): persisting the
marker in history fails the history scan; queuing the marker fails the
queued scan; queuing the marker and letting it promote fails the
post-drain history scan 3/3.
- `go test ./coderd -run 'TestChatMessageWithFiles/FileCapExceeded'
-count=100` and the full `TestChatMessageWithFiles` parent both pass.

> Mux acted on Mike's behalf to create this PR.

coderd/exp_chats_test.go

@@ -8299,8 +8299,6 @@ func TestChatMessageWithFiles(t *testing.T) {
 		extraResp, err := client.UploadChatFile(ctx, firstUser.OrganizationID, "image/png", "one-too-many.png", bytes.NewReader(pngData))
 		require.NoError(t, err)
 
-		messagesBefore, err := client.GetChatMessages(ctx, chat.ID, nil)
-		require.NoError(t, err)
 		_, err = client.CreateChatMessage(ctx, chat.ID, codersdk.CreateChatMessageRequest{
 			Content: []codersdk.ChatInputPart{
 				{Type: codersdk.ChatInputPartTypeText, Text: "one too many"},
@@ -8313,9 +8311,27 @@ func TestChatMessageWithFiles(t *testing.T) {
 		require.Equal(t, http.StatusBadRequest, sdkErr.StatusCode())
 		require.Contains(t, sdkErr.Message, "attachment limit")
 
-		messagesAfter, err := client.GetChatMessages(ctx, chat.ID, nil)
+		// getChatMessages reads history before queued messages, so a promotion
+		// can make one response miss the message in both places. Wait for the
+		// queue to empty, then read history again because promotion inserts a
+		// history row.
+		require.Eventually(t, func() bool {
+			m, err := client.GetChatMessages(ctx, chat.ID, nil)
+			return err == nil && len(m.QueuedMessages) == 0
+		}, testutil.WaitLong, testutil.IntervalMedium)
+
+		messages, err := client.GetChatMessages(ctx, chat.ID, nil)
 		require.NoError(t, err)
-		require.Len(t, messagesAfter.Messages, len(messagesBefore.Messages), "rejected send should not persist a message")
+		for _, msg := range messages.Messages {
+			for _, part := range msg.Content {
+				require.NotContains(t, part.Text, "one too many", "rejected send should not persist a message")
+			}
+		}
+		for _, queued := range messages.QueuedMessages {
+			for _, part := range queued.Content {
+				require.NotContains(t, part.Text, "one too many", "rejected send should not queue a message")
+			}
+		}
 		chatResult, err := client.GetChat(ctx, chat.ID)
 		require.NoError(t, err)
 		require.Len(t, chatResult.Files, codersdk.MaxChatFileIDs,