Atomic chat-cache writes + stop tests clobbering prod cache

Three intertwined fixes for the torn-write that corrupted the prod
data/chat-cache.json mid-session:

  - ChatCache.save / persistCooldown now write via temp file + rename.
    rename(2) is atomic on the same filesystem, so a crashed or
    concurrent writer can never leave NULs in the middle of the file.
    A small serialising write queue stops two concurrent saves from
    racing to the same temp name.

  - ChatCache throws if constructed without a filePath under NODE_ENV=test.
    The old default (./data/chat-cache.json) silently routed test
    writes into the production cache — exactly how the corruption
    happened. Future test that forgets to pass a tmp path fails loudly
    instead of stomping data.

  - Cooldown / delta-media / username-persist tests now pass an
    explicit tmp ChatCache so the new guard is satisfied.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/chat-service-cooldown.test.ts

@@ -23,7 +23,13 @@ function buildService() {
       new Response('{}', { status: 200, headers: { 'content-type': 'application/json' } }),
     )) as typeof fetch;
   const matrix = new MatrixClient(creds, logger, { minRequestIntervalMs: 0, fetchImpl });
-  const svc = new ChatService(matrix, logger);
+  // Point the cache at a tmp path; without this, the default falls back to
+  // ./data/chat-cache.json and the test would clobber the production cache.
+  const cache = new ChatCache(
+    logger,
+    path.join(os.tmpdir(), `chat-cache-${Date.now()}-${Math.random()}.json`),
+  );
+  const svc = new ChatService(matrix, logger, undefined, undefined, undefined, undefined, cache);
   return { svc, logger };
 }
 
@@ -168,7 +174,11 @@ describe('ChatService invitation cooldown', () => {
     matrix.sendTextMessage = async () => {
       throw new MatrixRateLimitError('429', 90_000, 'M_LIMIT_EXCEEDED', null);
     };
-    const svc = new ChatService(matrix, logger);
+    const cache = new ChatCache(
+      logger,
+      path.join(os.tmpdir(), `chat-cache-${Date.now()}-${Math.random()}.json`),
+    );
+    const svc = new ChatService(matrix, logger, undefined, undefined, undefined, undefined, cache);
 
     const result = await svc.sendChatMessage('!room:example.test', 'hi');
     expect(result).toBe(false);