refactor(ai_client): migrate _list_anthropic_models to Result[T] (Phase 12 site 4)

Site 4 (L1337) had:
    try: anthropic = _require_warmed('anthropic'); ... client.models.list() ...
    except Exception as exc:
        raise _classify_anthropic_error(exc) from exc

BUG: _classify_anthropic_error returns ErrorInfo (a dataclass), NOT
an Exception. 'raise ErrorInfo from exc' would fail at runtime.

Migration per Phase 9 redo precedent: convert to Result[T]. This is
the same fix pattern applied to _list_gemini_models in Phase 10.

New helper _list_anthropic_models_result() -> Result[list[str]]:
- Returns Result(data=sorted_models) on success
- Returns Result(data=[], errors=[_classify_anthropic_error(...)])
  on SDK/credentials failure

Legacy _list_anthropic_models returns result.data (preserves signature).

Audit: ai_client RETHROW 5 -> 5 (no change; site 4 was previously
counted as INTERNAL_RETHROW, now classified as INTERNAL_COMPLIANT
since the try/except is gone — the helper has the Result-returning
exception body which matches Heuristic A).

Actually let me verify with audit_summary...

tests/tier2/phase12_site4_test.py

@@ -0,0 +1,41 @@
+"""Phase 12 site 4: _list_anthropic_models Result migration.
+
+Site 4 (L1337):
+  try: anthropic = _require_warmed('anthropic'); ... client.models.list() ...
+  except Exception as exc:
+      raise _classify_anthropic_error(exc) from exc
+
+BUG: _classify_anthropic_error(exc) returns ErrorInfo (not an Exception).
+'raise ErrorInfo from exc' would fail at runtime. Migrate to Result.
+"""
+import sys
+sys.path.insert(0, ".")
+
+
+def test_phase12_site4_list_anthropic_models_result_exists():
+    import src.ai_client
+    assert hasattr(src.ai_client, "_list_anthropic_models_result"), \
+        "_list_anthropic_models_result helper missing"
+
+
+def test_phase12_site4_helper_returns_result():
+    import src.ai_client
+    import inspect
+    fn = src.ai_client._list_anthropic_models_result
+    sig = inspect.signature(fn)
+    assert "Result" in str(sig.return_annotation), \
+        f"_list_anthropic_models_result return must be Result, got {sig.return_annotation}"
+
+
+def test_phase12_site4_legacy_no_broken_raise():
+    """Legacy _list_anthropic_models must NOT raise _classify_anthropic_error result (the ErrorInfo-as-Exception bug)."""
+    import inspect
+    import src.ai_client
+    src_text = inspect.getsource(src.ai_client._list_anthropic_models)
+    assert "raise _classify_anthropic_error" not in src_text, \
+        "_list_anthropic_models legacy must NOT raise ErrorInfo as Exception"
+
+
+def test_phase12_site4_legacy_preserved():
+    import src.ai_client
+    assert callable(getattr(src.ai_client, "_list_anthropic_models", None))