fix(fusion_helpdesk_central): engagement now posts public chatter for employee

Sending an engagement triggered template.send_mail(), which logged the
outbound email to the chatter as a `notification` message with the
internal `Note` subtype. That's correct for nexa-side bookkeeping (we
don't want the raw email body propagating to the customer), but it
meant nothing public was posted — so the entech-side My Tickets inbox
showed no activity. The employee couldn't tell their request had been
escalated for approval.

_fc_reset_engagement now posts a follow-up public message via
message_post (subtype mail.mt_comment, message_type='comment') with:

  ⏳ Awaiting owner approval from <owner_name>.
  Their decision will appear here when they reply.

  Our reply:
  > <findings text>

This survives the entech _public_messages filter (comment +
non-internal subtype) and propagates to the employee's My Tickets
thread, giving them context AND the engineer's reply without exposing
the raw outbound email or the owner's email address.

Smoke-tested live on ticket #54: re-engaged with the same owner, the
new mail.message (id=348213) is subtype=Discussions / internal=False /
message_type=comment, and contains both the awaiting-approval notice
and the findings text. _public_messages would surface it.

Bumps fusion_helpdesk_central to 19.0.2.4.1.

fusion_helpdesk_central/__manifest__.py

@@ -3,7 +3,7 @@
 # License OPL-1
 {
     'name': 'Fusion Helpdesk Central — Client API Keys',
-    'version': '19.0.2.4.0',
+    'version': '19.0.2.4.1',
     'category': 'Productivity',
     'summary': 'Admin UI on the central Odoo for issuing per-client API '
                'keys used by fusion_helpdesk client deployments.',