refactor(ai_client): migrate get_token_stats count_tokens to Result[int] (Phase 11 sites 9+10)

Both sites 9 (gemini) and 10 (gemini_cli) in get_token_stats had:
  try: _ensure_gemini_client()
       if _gemini_client:
           resp = _gemini_client.models.count_tokens(model=_model, contents=md_content)
           total_tokens = cast(int, resp.total_tokens)
  except Exception: pass

Body: pass = SS violation.

New helper _count_gemini_tokens_for_stats_result(md_content) -> Result[int]:
- Returns Result(data=token_count) on success
- Returns Result(data=0, errors=[ErrorInfo]) on SDK failure or warmup failure
- Caller treats 0 as 'token count unavailable' and falls back to
  character-based estimation

Legacy get_token_stats now uses:
  if p in ('gemini', 'gemini_cli'):
      total_tokens = _count_gemini_tokens_for_stats_result(md_content).data

(combined both branches into one since the logic was identical)

Audit: ai_client SS 5 -> 3. COMPLIANT 31 -> 32.

tests/tier2/phase11_sites910_test.py

@@ -0,0 +1,35 @@
+"""Phase 11 sites 9+10: get_token_stats count_tokens (gemini + gemini_cli).
+
+Both have:
+    try:
+        _ensure_gemini_client()
+        if _gemini_client:
+            resp = _gemini_client.models.count_tokens(model=_model, contents=md_content)
+            total_tokens = cast(int, resp.total_tokens)
+    except Exception:
+        pass
+
+Body: pass = SS violation. Migrate via Result[int] helper.
+"""
+import sys
+sys.path.insert(0, ".")
+
+
+def test_phase11_sites910_count_gemini_tokens_for_stats_result_exists():
+    import src.ai_client
+    assert hasattr(src.ai_client, "_count_gemini_tokens_for_stats_result"), \
+        "_count_gemini_tokens_for_stats_result helper missing"
+
+
+def test_phase11_sites910_helper_returns_result():
+    import src.ai_client
+    import inspect
+    fn = src.ai_client._count_gemini_tokens_for_stats_result
+    sig = inspect.signature(fn)
+    assert "Result" in str(sig.return_annotation), \
+        f"_count_gemini_tokens_for_stats_result return must be Result, got {sig.return_annotation}"
+
+
+def test_phase11_sites910_get_token_stats_legacy_preserved():
+    import src.ai_client
+    assert callable(getattr(src.ai_client, "get_token_stats", None))