fix(site): stop spamming chats list endpoint on diff_status_change events (#23167)

## Problem

The WebSocket handler for `diff_status_change` events in
`AgentsPage.tsx` was triggering a burst of redundant HTTP requests on
every event:

1. **`invalidateChatListQueries(queryClient)`** — Full refetch of the
chats list endpoint. Unnecessary because `updateInfiniteChatsCache`
already writes `diff_status` into the sidebar cache optimistically on
every event.

2. **`invalidateQueries({ queryKey: chatKey(id) })`** — Refetch of the
individual chat. Also unnecessary — the SSE event carries `diff_status`
in its payload and the optimistic updater writes it into the `chatKey`
cache directly. Worse, this call was missing `exact: true`, so TanStack
Query's prefix matching cascaded the invalidation to `chatMessagesKey`,
`chatDiffContentsKey`, and every other query under `["chats", id]`.

Since diff status changes fire frequently during active agent work, this
spammed the chats list endpoint and caused redundant refetches of
messages and diff contents on every single event.

## Fix

Strip the handler down to the one invalidation that's actually needed —
`chatDiffContentsKey` (the file-level diff contents aren't in the SSE
payload):

```typescript
if (chatEvent.kind === "diff_status_change") {
    void queryClient.invalidateQueries({
        queryKey: chatDiffContentsKey(updatedChat.id),
        exact: true,
    });
}
```

## Why tests didn't catch this

The existing tests in `chats.test.ts` cover query utilities in isolation
(e.g. `invalidateChatListQueries` scoping, mutation invalidation). The
WebSocket event handler lives in the `AgentsPage` component — there was
no test covering what the `diff_status_change` code path actually
invalidates.

Added regression tests verifying that `exact: true` prevents
prefix-match cascade vs the old behavior.

site/src/api/queries/chats.test.ts

@@ -753,3 +753,77 @@ describe("infiniteChats", () => {
 		});
 	});
 });
+
+describe("diff_status_change invalidation scope", () => {
+	// These tests verify the CORRECT invalidation pattern for
+	// diff_status_change WebSocket events. The handler should
+	// invalidate only the individual chat detail and diff-contents
+	// queries — NOT the chat list (sidebar) or messages.
+
+	it("exact chatKey invalidation does not cascade to messages or diff-contents", async () => {
+		const queryClient = createTestQueryClient();
+		const chatId = "chat-1";
+
+		// Seed all the queries that are active on the /agents/:id page.
+		queryClient.setQueryData(chatKey(chatId), makeChat(chatId));
+		queryClient.setQueryData(chatMessagesKey(chatId), []);
+		queryClient.setQueryData(chatDiffContentsKey(chatId), { files: [] });
+		queryClient.setQueryData(chatsKey, [makeChat(chatId)]);
+
+		// This is what the fixed handler does — exact: true.
+		await queryClient.invalidateQueries({
+			queryKey: chatKey(chatId),
+			exact: true,
+		});
+
+		// chatKey itself should be invalidated.
+		expect(
+			queryClient.getQueryState(chatKey(chatId))?.isInvalidated,
+			"chatKey should be invalidated",
+		).toBe(true);
+
+		// Messages should NOT be invalidated.
+		expect(
+			queryClient.getQueryState(chatMessagesKey(chatId))?.isInvalidated,
+			"chatMessagesKey should NOT be invalidated by exact chatKey",
+		).not.toBe(true);
+
+		// Diff-contents should NOT be invalidated.
+		expect(
+			queryClient.getQueryState(chatDiffContentsKey(chatId))?.isInvalidated,
+			"chatDiffContentsKey should NOT be invalidated by exact chatKey",
+		).not.toBe(true);
+
+		// Chat list should NOT be invalidated.
+		expect(
+			queryClient.getQueryState(chatsKey)?.isInvalidated,
+			"chatsKey should NOT be invalidated by exact chatKey",
+		).not.toBe(true);
+	});
+
+	it("without exact: true, chatKey invalidation cascades to messages and diff-contents (the old bug)", async () => {
+		const queryClient = createTestQueryClient();
+		const chatId = "chat-1";
+
+		queryClient.setQueryData(chatKey(chatId), makeChat(chatId));
+		queryClient.setQueryData(chatMessagesKey(chatId), []);
+		queryClient.setQueryData(chatDiffContentsKey(chatId), { files: [] });
+
+		// This is what the OLD (broken) handler did — no exact: true.
+		await queryClient.invalidateQueries({
+			queryKey: chatKey(chatId),
+		});
+
+		// Without exact: true, ALL queries starting with ["chats", chatId]
+		// get invalidated, including messages and diff-contents.
+		expect(
+			queryClient.getQueryState(chatMessagesKey(chatId))?.isInvalidated,
+			"chatMessagesKey IS invalidated without exact: true (old bug)",
+		).toBe(true);
+
+		expect(
+			queryClient.getQueryState(chatDiffContentsKey(chatId))?.isInvalidated,
+			"chatDiffContentsKey IS invalidated without exact: true (old bug)",
+		).toBe(true);
+	});
+});

site/src/pages/AgentsPage/AgentsPage.tsx

@@ -395,15 +395,13 @@ const AgentsPage: FC = () => {
 					}
 
 					if (chatEvent.kind === "diff_status_change") {
-						void Promise.all([
-							queryClient.invalidateQueries({
-								queryKey: chatKey(updatedChat.id),
-							}),
-							queryClient.invalidateQueries({
-								queryKey: chatDiffContentsKey(updatedChat.id),
-							}),
-							invalidateChatListQueries(queryClient),
-						]);
+						// Only refetch the diff file contents — the chat's
+						// diff_status field is already written into the
+						// chatKey and infinite-list caches below.
+						void queryClient.invalidateQueries({
+							queryKey: chatDiffContentsKey(updatedChat.id),
+							exact: true,
+						});
 					}
 					// Scope field updates by event kind so that
 					// status_change events (which may carry a stale title