refactor(ai_client): extract _delete_gemini_cache_result helper (Phase 10 sites 2+3)

Sites L1680 (cache.delete on context change) and L1692 (cache.delete on
TTL expiry) had identical patterns:
  try: _gemini_client.caches.delete(name=_gemini_cache.name)
  except Exception as e: _append_comms('OUT', 'request', {'message': f'[CACHE DELETE WARN] {e}'})

Per TIER1_REVIEW: logging is NOT a drain. MIGRATE to Result[T].

Single helper _delete_gemini_cache_result() -> Result[None]:
- Returns Result(data=None) on success
- Returns Result(data=None, errors=[ErrorInfo]) on SDK failure + logs warning to comms
- Caller (_send_gemini) ignores errors (best-effort cleanup)

Audit: ai_client BC 8 -> 6. Both sites migrated.

tests/tier2/phase10_site23_test.py

@@ -0,0 +1,26 @@
+"""Phase 10 invariant tests (RED) — sites 2+3: _delete_gemini_cache_result.
+
+Sites 2 (L1680) and 3 (L1692): both are
+    try: _gemini_client.caches.delete(name=_gemini_cache.name)
+    except Exception as e: _append_comms("OUT", "request", {"message": f"[CACHE DELETE WARN] {e}"})
+
+Migrate via single helper _delete_gemini_cache_result() -> Result[None].
+"""
+import sys
+sys.path.insert(0, ".")
+
+
+def test_phase10_site23_delete_gemini_cache_result_exists():
+    import src.ai_client
+    assert hasattr(src.ai_client, "_delete_gemini_cache_result"), \
+        "_delete_gemini_cache_result helper missing"
+
+
+def test_phase10_site23_delete_gemini_cache_result_returns_result():
+    """The helper must return Result[None]."""
+    import src.ai_client
+    import inspect
+    fn = src.ai_client._delete_gemini_cache_result
+    sig = inspect.signature(fn)
+    assert "Result" in str(sig.return_annotation), \
+        f"_delete_gemini_cache_result return must be Result, got {sig.return_annotation}"