fix(gateway): internal-only sessions still pin the session-context prompt

The internal-event branch returned an unpinned render when no pin existed
yet, so sessions that only ever see internal turns (kanban/API-server
wakes, startup resumes) re-rendered the session-context prompt every turn
and lost the verbatim-reuse immunity base gave them. Reuse an existing pin
when present; otherwise fall through to render-and-pin as before.

Applies teknium1's inline review suggestion on #122048.

gateway/run_agent_cache.py

@@ -629,11 +629,11 @@ class GatewayAgentCacheMixin:
         source rebuilt from the persisted origin, without chat_name/user_name/message_id. Rendering
         from it re-keyed the pin, and the next human turn re-keyed it back (A→B→A), rewriting
         already-sent system bytes each time. An internal event is never a real metadata change, so
-        it reuses the existing pin verbatim and never re-pins."""
+        it reuses an existing pin verbatim; with no pin yet it renders and pins as usual."""
         _pin_state = self._peek_session_state(session_key) if session_key else None
         _eph_pin = _pin_state.conversation.ephemeral_pin if _pin_state else None
-        if internal:
-            return _eph_pin[1] if _eph_pin is not None else build_session_context_prompt(context, redact_pii=redact_pii)
+        if internal and _eph_pin is not None:
+            return _eph_pin[1]
         _eph_key = self._ephemeral_change_key(context, redact_pii)
         if _eph_pin is not None and _eph_pin[0] == _eph_key:
             return _eph_pin[1]