fix(server): remove a NUL byte from the fatal-error test file and an unref hazard on the exit path

The test file had one literal NUL byte in a string literal, so Git read
the whole file as binary and hid it from every diff. The fallback
string was unreachable, so the fix asserts on `error.stack` directly.

`flushWithTimeout` also called `timer.unref?.()`. An unreferenced timer
does not keep the event loop open, so a crash with no other open
handle could let Node exit with code 0 before `process.exit(1)` runs.
The 2-second bound already gives the same protection, so the line is
removed, and the timer now clears inside `finish` instead.

Co-authored-by: Paperclip <noreply@paperclip.ing>

server/src/fatal-error-handler.ts

@@ -130,10 +130,10 @@ function flushWithTimeout(client: TelemetryClient, timeoutMs: number): Promise<v
     const finish = () => {
       if (settled) return;
       settled = true;
+      clearTimeout(timer);
       resolve();
     };
     const timer = setTimeout(finish, timeoutMs);
-    timer.unref?.();
     client.flush().then(finish, finish);
   });
 }