feat: surface in-dialog hold/un-hold to the application (#1564)

Applications currently have no way to observe a mid-call hold/un-hold.
The feature-server handles the in-dialog re-INVITE internally (re-modifies
the endpoint, returns 200) but never notifies the app, so a transcription
or agent-assist app can't pause/resume when a caller is held or retrieved.

This detects the hold state transition on a re-INVITE and delivers it over
the existing sipRequestWithinDialogHook channel as a verb:hook, plus emits
a local 'hold'/'unhold' event so in-process tasks (e.g. transcribe) can react.

- call-session.js: ordinary calls — reuse isOnhold() (a=sendonly/a=inactive).
- siprec-call-session.js: SIPREC overrides _onReinvite, so a separate handler
  is required. SIPREC recording streams are sendonly by nature, so a stream
  flipping to a=inactive is treated as the hold signal; the full multipart
  body (SDP + rs-metadata) is always forwarded. Fires on transition only;
  other re-INVITEs are delivered as event:"reinvite".

Delivery uses requestor.request('verb:hook', ...) — the same channel
_handleRefer uses — rather than a task-bound performHook, so events are not
dropped if the current task has ended. Enable per call with:
  {"verb":"config","sipRequestWithinDialogHook":"/hold-events"}

Confirmed live against Cisco CUBE (Cisco-SIPGateway/IOS-17.12.7b) over SIPREC:
both recording streams flip sendonly<->inactive on hold/resume, each
transition emitted correctly.

The whole feature is opt-in behind the JAMBONES_HOLD_UNHOLD_EVENTS env var;
when it is unset, _notifyHoldState / _notifySipRecReinvite are no-ops (neither
the local event nor the hook fires).

Reinstates the feature previously reverted in #1563. The revert was due to the
unit test failing in the full suite: lib/config.js captures the env var once at
load time, and earlier suite files require config/session modules (flag unset)
before this test could set it, so the enabled-path assertions saw a disabled
feature. The test now force-reloads config + the session modules with the flag
set (and restores the module cache afterward), so it is robust to suite load order.

lib/config.js

@@ -71,6 +71,7 @@ const OTEL_EXPORTER_COLLECTOR_URL = process.env.OTEL_EXPORTER_COLLECTOR_URL;
 
 const JAMBONES_LOGLEVEL = process.env.JAMBONES_LOGLEVEL || 'info';
 const JAMBONES_INJECT_CONTENT = process.env.JAMBONES_INJECT_CONTENT;
+const JAMBONES_HOLD_UNHOLD_EVENTS = process.env.JAMBONES_HOLD_UNHOLD_EVENTS;
 
 const PORT = parseInt(process.env.HTTP_PORT, 10) || 3000;
 const HTTP_IP = process.env.HTTP_IP;
@@ -170,6 +171,7 @@ module.exports = {
   JAMBONES_API_BASE_URL,
   JAMBONES_TIME_SERIES_HOST,
   JAMBONES_INJECT_CONTENT,
+  JAMBONES_HOLD_UNHOLD_EVENTS,
   JAMBONES_EAGERLY_PRE_CACHE_AUDIO,
   JAMBONES_ESL_LISTEN_ADDRESS,
   JAMBONES_SBCS,