fix: heartbeat _getActiveHandles optional chain bug (Qodo #2)

Qodo correctly flagged `_getActiveHandles?.().length` as a latent
TypeError: `?.()` guards the call but the call's `undefined` return
on a missing method still hits `.length`, which throws. Since the
heartbeat fires on a setInterval inside the mocha bootstrap, a Node
build without the underscore-prefixed internals would take down the
whole backend test run.

Capture the array first, then read `.length` only when it actually
exists. -1 stays as the "API missing" sentinel.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: JohnMcLear

src/tests/backend/diagnostics.ts

@@ -71,8 +71,15 @@ diag('diagnostics loaded');
 // tightly we can bracket the kill timestamp.
 const heartbeat = setInterval(() => {
   const mem = process.memoryUsage();
-  const handles = (process as any)._getActiveHandles?.().length ?? -1;
-  const requests = (process as any)._getActiveRequests?.().length ?? -1;
+  // _getActiveHandles / _getActiveRequests are undocumented Node internals.
+  // The earlier shape `_getActiveHandles?.().length ?? -1` was a bug: `?.()`
+  // only guards the call, so a missing method returns `undefined` and then
+  // `.length` throws TypeError — which would take down the whole test run.
+  // Capture the array first, then read .length only when it actually exists.
+  const handlesArr = (process as any)._getActiveHandles?.();
+  const handles = handlesArr ? handlesArr.length : -1;
+  const requestsArr = (process as any)._getActiveRequests?.();
+  const requests = requestsArr ? requestsArr.length : -1;
   diag(`hb running="${currentTest}" lastFinished="${lastFinishedTest}" ` +
     `rss=${Math.round(mem.rss / 1024 / 1024)}M ` +
     `heap=${Math.round(mem.heapUsed / 1024 / 1024)}M ` +