fix: address Copilot review round 3 on #2318

- fetchTracking: the close report is guarded so a throwing listener
  cannot become an unhandled rejection off the discarded watcher; a
  bare CR that is the last byte of the stream terminates its event; a
  moving event count is coalesced to one report per interval (default
  250ms, 0 in tests), with the open and the close still immediate.
- MessageTrackingTransport rolls back a request whose signal was already
  aborted before the send began, and suppresses the rollback only for an
  abort that lands during the send.
- useTickingClock takes an `enabled` flag; Connection Info installs no
  interval when there is no activity section.
- The diagnostics-callbacks test justifies its private-seam cast.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01APKSYHGxXBUpnR7ErFiLJn
Signed-off-by: cliffhall <cliff@futurescale.com>

clients/web/src/components/groups/ConnectionInfoContent/ConnectionInfoContent.tsx

@@ -337,8 +337,13 @@ export function ConnectionInfoContent({
     : SERVER_INFO_NOT_REPORTED_LABEL;
 
   // The activity rows' clock: the snapshot's own on first paint (pure), then
-  // the wall clock once a second so "sent 5s ago" keeps counting.
-  const now = useTickingClock(diagnostics?.capturedAt ?? 0);
+  // the wall clock once a second so "sent 5s ago" keeps counting. No timer
+  // at all when there is no activity section to tick.
+  const now = useTickingClock(
+    diagnostics?.capturedAt ?? 0,
+    1000,
+    diagnostics !== undefined,
+  );
 
   const serverCaps = getServerCapabilityEntries(capabilities, protocolEra);
   const clientCaps = getCapabilityEntries(