fix(media): repair JSON_CONTAINS auto-routing + reconcile stale backend tests

Real bug:
- capability_registry.resolve_auto_model and chat_media._build_media_route used
  `Column.op("JSON_CONTAINS")(col, val)`, but SQLAlchemy's op() accepts only ONE
  argument -> TypeError on every image/video/audio auto-route, silently failing
  media generation (caught as "后台生成异常"). Replaced with func.json_contains().

Test reconciliation (no production behavior change):
- tests/test_context_engine.py: tiktoken upgrade made the hardcoded len//2 token
  estimates wrong; relax to tokenizer-agnostic assertions and add
  structured_memory=None to the _Summary mock (code now branches on it).
- tests/test_regression_r1r2r3.py: R3 FakeModel lacked `name` (code reads m.name);
  also the assertion assumed a literal '超时' string, but the failover architecture
  catches model-level exceptions and writes a user-understandable error marker
  instead. Align the assertion to the real contract: a media failure/timeout must
  be written back as a user-understandable error message, never left as '生成中…'.

CI gate: 28 backend pytest + 15 frontend vitest all green.

backend/python/app/api/chat_media.py

@@ -893,7 +893,7 @@ def _build_media_route(db, capability: str, preferred_id: str = None):
 
     others = db.query(_AM).filter(
         _AM.is_enabled == True,
-        _AM.capabilities.op("JSON_CONTAINS")(_AM.capabilities, f'"{capability}"'),
+        func.json_contains(_AM.capabilities, f'"{capability}"'),
     ).order_by(_AM.is_default.desc(), _AM.created_at.asc()).all()
     for m in others:
         _add(m)