perf(coderd/database): skip redundant chat row update in InsertChatMessage (#23111)

## Summary

- add an `IS DISTINCT FROM` guard to `InsertChatMessage`'s
`updated_chat` CTE so `chats.last_model_config_id` is only rewritten
when the incoming `model_config_id` actually changes
- regenerate the query layer
- add focused regression coverage for the two meaningful behaviors:
same-model inserts and real model switches
- trim redundant message-field assertions so the new test stays focused
on the guard behavior

## Proof this is an improvement

This PR reduces work in the hottest chat write query without changing
the insert behavior.

### Why the old query did unnecessary work

Before this change, `InsertChatMessage` always ran this update whenever
`model_config_id` was non-null:

```sql
UPDATE chats
SET last_model_config_id = sqlc.narg('model_config_id')::uuid
WHERE id = @chat_id::uuid
  AND sqlc.narg('model_config_id')::uuid IS NOT NULL
```

That means the query rewrote the `chats` row even when
`chats.last_model_config_id` was already equal to the incoming value.

### What changes in this PR

This PR adds:

```sql
AND chats.last_model_config_id IS DISTINCT FROM sqlc.narg('model_config_id')::uuid
```

So same-model inserts still insert the message, but they no longer
perform a redundant `UPDATE chats`.

### Why this matters on the hot path

From the chat scaletest investigation that motivated this change:

- `InsertChatMessage` (+ `updated_chat` CTE) was the hottest write query
- about **104k calls**
- about **0.69 ms average latency**
- about **71.8 s total DB execution time**

We also verified common callsites where the update is provably
redundant:

- `CreateChat` inserts the chat with `LastModelConfigID =
opts.ModelConfigID`, then immediately inserts initial system/user
messages with that same model config
- follow-up user messages commonly pass `lockedChat.LastModelConfigID`
straight into `InsertChatMessage`
- assistant/tool/summary persistence keeps the current model in the
common case; only real switches or fallback cases need the chat row
update

That means a meaningful fraction of executions of the hottest DB write
query move from:

- **before:** insert message **+** rewrite chat row
- **after:** insert message only

This should reduce row churn and write contention on `chats`, especially
against other chat-row writers like `UpdateChatStatus` and
`GetChatByIDForUpdate`.

coderd/database/querier_test.go

@@ -8997,6 +8997,131 @@ func TestInsertWorkspaceAgentDevcontainers(t *testing.T) {
 	}
 }
 
+func TestInsertChatMessage(t *testing.T) {
+	t.Parallel()
+
+	insertModelConfig := func(
+		t *testing.T,
+		store database.Store,
+		ctx context.Context,
+		userID uuid.UUID,
+		provider string,
+		model string,
+		displayName string,
+		isDefault bool,
+	) database.ChatModelConfig {
+		t.Helper()
+
+		modelConfig, err := store.InsertChatModelConfig(ctx, database.InsertChatModelConfigParams{
+			Provider:             provider,
+			Model:                model,
+			DisplayName:          displayName,
+			CreatedBy:            uuid.NullUUID{UUID: userID, Valid: true},
+			UpdatedBy:            uuid.NullUUID{UUID: userID, Valid: true},
+			Enabled:              true,
+			IsDefault:            isDefault,
+			ContextLimit:         128000,
+			CompressionThreshold: 80,
+			Options:              json.RawMessage(`{}`),
+		})
+		require.NoError(t, err)
+
+		return modelConfig
+	}
+
+	setupChat := func(t *testing.T) (database.Store, context.Context, database.User, database.Chat, string, database.ChatModelConfig) {
+		t.Helper()
+
+		store, _ := dbtestutil.NewDB(t)
+		ctx := context.Background()
+
+		dbgen.Organization(t, store, database.Organization{})
+		user := dbgen.User(t, store, database.User{})
+		provider := "openai"
+
+		_, err := store.InsertChatProvider(ctx, database.InsertChatProviderParams{
+			Provider:    provider,
+			DisplayName: "OpenAI",
+			APIKey:      "test-key",
+			Enabled:     true,
+		})
+		require.NoError(t, err)
+
+		modelConfigA := insertModelConfig(
+			t,
+			store,
+			ctx,
+			user.ID,
+			provider,
+			"test-model-a-"+uuid.NewString(),
+			"Test Model A",
+			true,
+		)
+
+		chat, err := store.InsertChat(ctx, database.InsertChatParams{
+			OwnerID:           user.ID,
+			LastModelConfigID: modelConfigA.ID,
+			Title:             "test-chat-" + uuid.NewString(),
+		})
+		require.NoError(t, err)
+
+		return store, ctx, user, chat, provider, modelConfigA
+	}
+
+	insertMessage := func(t *testing.T, store database.Store, ctx context.Context, chatID, userID, modelConfigID uuid.UUID, content string) {
+		t.Helper()
+
+		_, err := store.InsertChatMessage(ctx, database.InsertChatMessageParams{
+			ChatID:         chatID,
+			CreatedBy:      uuid.NullUUID{UUID: userID, Valid: true},
+			ModelConfigID:  uuid.NullUUID{UUID: modelConfigID, Valid: true},
+			Role:           database.ChatMessageRoleUser,
+			ContentVersion: chatprompt.CurrentContentVersion,
+			Visibility:     database.ChatMessageVisibilityBoth,
+			Content: pqtype.NullRawMessage{
+				RawMessage: json.RawMessage(fmt.Sprintf("%q", content)),
+				Valid:      true,
+			},
+		})
+		require.NoError(t, err)
+	}
+
+	t.Run("ModelSwitchUpdatesLastModelConfigID", func(t *testing.T) {
+		t.Parallel()
+
+		store, ctx, user, chat, provider, modelConfigA := setupChat(t)
+		modelConfigB := insertModelConfig(
+			t,
+			store,
+			ctx,
+			user.ID,
+			provider,
+			"test-model-b-"+uuid.NewString(),
+			"Test Model B",
+			false,
+		)
+
+		insertMessage(t, store, ctx, chat.ID, user.ID, modelConfigB.ID, "switch models")
+
+		gotChat, err := store.GetChatByID(ctx, chat.ID)
+		require.NoError(t, err)
+		require.Equal(t, modelConfigA.ID, chat.LastModelConfigID)
+		require.Equal(t, modelConfigB.ID, gotChat.LastModelConfigID)
+	})
+
+	t.Run("SameModelDoesNotBreakAnything", func(t *testing.T) {
+		t.Parallel()
+
+		store, ctx, user, chat, _, modelConfigA := setupChat(t)
+
+		insertMessage(t, store, ctx, chat.ID, user.ID, modelConfigA.ID, "same model")
+
+		gotChat, err := store.GetChatByID(ctx, chat.ID)
+		require.NoError(t, err)
+		require.Equal(t, modelConfigA.ID, gotChat.LastModelConfigID)
+	})
+}
+
 func TestGetChatMessagesForPromptByChatID(t *testing.T) {
 	t.Parallel()
 

coderd/database/queries.sql.go

@@ -4211,6 +4211,7 @@ WITH updated_chat AS (
     WHERE
         id = $1::uuid
         AND $3::uuid IS NOT NULL
+        AND chats.last_model_config_id IS DISTINCT FROM $3::uuid
 )
 INSERT INTO chat_messages (
     chat_id,

coderd/database/queries/chats.sql

@@ -165,6 +165,7 @@ WITH updated_chat AS (
     WHERE
         id = @chat_id::uuid
         AND sqlc.narg('model_config_id')::uuid IS NOT NULL
+        AND chats.last_model_config_id IS DISTINCT FROM sqlc.narg('model_config_id')::uuid
 )
 INSERT INTO chat_messages (
     chat_id,