refactor(ai_client): migrate cleanup + reset_session cache.delete to helper (Phase 11 sites 3+4)

Sites L432 (cleanup) and L450 (reset_session) had:
    try: _gemini_client.caches.delete(name=_gemini_cache.name)
    except Exception: pass

This is bare 'except: pass' = INTERNAL_SILENT_SWALLOW violation (logging is NOT
a drain; 'pass' is the worst form of silent recovery).

Migration: use existing _delete_gemini_cache_result() helper (added Phase 10).
The helper returns Result[None]; on SDK error logs a warning to comms.
The caller ignores the Result (cleanup is best-effort).

Audit: ai_client SS 11 -> 9.

src/ai_client.py

@@ -426,11 +426,7 @@ def cleanup() -> None:
  """Performs cleanup operations like deleting server-side Gemini caches."""
  global _gemini_client,    _gemini_cache, _gemini_cached_file_paths
  if     _gemini_client and _gemini_cache:
-  #TODO(Ed): Exception(Review)
-  try:
-   _gemini_client.caches.delete(name=_gemini_cache.name)
-  except Exception:
-   pass
+  _delete_gemini_cache_result()
  _gemini_cached_file_paths = []
 
 def reset_session() -> None:
@@ -444,11 +440,7 @@ def reset_session() -> None:
  global _CACHED_ANTHROPIC_TOOLS, _CACHED_DEEPSEEK_TOOLS
  global _gemini_cli_adapter
  if _gemini_client and _gemini_cache:
-  #TODO(Ed): Review(Exception)
-  try:
-   _gemini_client.caches.delete(name=_gemini_cache.name)
-  except Exception:
-   pass
+  _delete_gemini_cache_result()
  _gemini_client            = None
  _gemini_chat              = None
  _gemini_cache             = None