fix(site/src/pages/AgentsPage): stabilize remote diff cache keys (#23487)

## Summary
- use React Query's `dataUpdatedAt` as the remote diff cache
invalidation token instead of a component-local counter
- keep the `@pierre/diffs` cache key stable across remounts without a
custom hashing implementation
- preserve targeted coverage for the cache-key helper used by the
/agents remote diff viewer

## Testing
- `cd site && pnpm exec biome check
src/pages/AgentsPage/components/DiffViewer/RemoteDiffPanel.tsx
src/pages/AgentsPage/components/DiffViewer/diffCacheKey.ts
src/pages/AgentsPage/components/DiffViewer/diffCacheKey.test.ts`
- `cd site && pnpm exec vitest run
src/pages/AgentsPage/components/DiffViewer/diffCacheKey.test.ts
--project=unit`
- `cd site && pnpm exec tsc -p .`

site/src/pages/AgentsPage/components/DiffViewer/RemoteDiffPanel.tsx

@@ -19,6 +19,7 @@ import type { ChatMessageInputRef } from "../AgentChatInput";
 import { CommentableDiffViewer } from "../DiffViewer/CommentableDiffViewer";
 import { DiffStatBadge } from "../DiffViewer/DiffStats";
 import type { DiffStyle } from "../DiffViewer/DiffViewer";
+import { getDiffCacheKeyPrefix } from "../DiffViewer/diffCacheKey";
 
 export { InlinePromptInput } from "../DiffViewer/CommentableDiffViewer";
 
@@ -89,33 +90,22 @@ export const RemoteDiffPanel: FC<RemoteDiffPanelProps> = ({
 	});
 
 	const diffContent = diffContentsQuery.data?.diff;
-	const [diffVersion, setDiffVersion] = useState(0);
-	const [prevDiffContent, setPrevDiffContent] = useState<string | undefined>(
-		undefined,
-	);
-	if (diffContent !== prevDiffContent) {
-		setPrevDiffContent(diffContent);
-		setDiffVersion((v) => v + 1);
-	}
-
 	const parsedFiles = (() => {
 		if (!diffContent) {
 			return [] as FileDiffMetadata[];
 		}
 		try {
-			// The cacheKeyPrefix enables the worker pool's LRU cache
-			// so highlighted ASTs are reused across re-renders instead
-			// of being re-computed on every render cycle. We include a
-			// version counter derived from the diff content so that when
-			// the diff changes (e.g. new commits pushed) the old cached
-			// highlight AST is not reused with mismatched line indices,
-			// which would cause DiffHunksRenderer.processDiffResult to
-			// throw. Unlike dataUpdatedAt, this counter only increments
-			// when the actual diff string changes, avoiding unnecessary
-			// recomputation on refetches with identical content.
+			// The @pierre/diffs worker pool only keys cached highlighted
+			// ASTs by `cacheKey`, so the key must change whenever the diff
+			// query updates. React Query's `dataUpdatedAt` survives panel
+			// remounts, which prevents stale cache hits from pairing a new
+			// FileDiffMetadata with an older highlighted AST.
 			const patches = parsePatchFiles(
 				diffContent,
-				`chat-${chatId}-v${diffVersion}`,
+				getDiffCacheKeyPrefix(
+					`chat-${chatId}`,
+					diffContentsQuery.dataUpdatedAt,
+				),
 			);
 			return patches.flatMap((p) => p.files);
 		} catch {

site/src/pages/AgentsPage/components/DiffViewer/diffCacheKey.test.ts

@@ -0,0 +1,21 @@
+import { getDiffCacheKeyPrefix } from "./diffCacheKey";
+
+describe("getDiffCacheKeyPrefix", () => {
+	it("returns the same key for the same scope and query update time", () => {
+		expect(getDiffCacheKeyPrefix("chat-123", 101)).toBe(
+			getDiffCacheKeyPrefix("chat-123", 101),
+		);
+	});
+
+	it("changes when the query update time changes", () => {
+		expect(getDiffCacheKeyPrefix("chat-123", 101)).not.toBe(
+			getDiffCacheKeyPrefix("chat-123", 202),
+		);
+	});
+
+	it("changes when the scope changes", () => {
+		expect(getDiffCacheKeyPrefix("chat-123", 101)).not.toBe(
+			getDiffCacheKeyPrefix("chat-456", 101),
+		);
+	});
+});

site/src/pages/AgentsPage/components/DiffViewer/diffCacheKey.ts

@@ -0,0 +1,12 @@
+/**
+ * Build a stable worker-pool cache key prefix for `@pierre/diffs`.
+ *
+ * We use React Query's `dataUpdatedAt` as the invalidation token instead of a
+ * component-local counter. That timestamp survives component remounts, so a
+ * freshly fetched diff cannot accidentally reuse a highlighted AST cached for
+ * an older diff body.
+ */
+export const getDiffCacheKeyPrefix = (
+	prefix: string,
+	dataUpdatedAt: number,
+): string => `${prefix}-${dataUpdatedAt}`;