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/capability_registry.py

@@ -98,12 +98,10 @@ def resolve_auto_model(capability: str, db: Session) -> Optional[AIModel]:
     # 媒体类能力(image/video/audio)：无显式配置时自动发现任一启用的模型
     # 避免用户未在后台配置"能力默认模型"时 Auto 回退到文本模型导致"无法生成图片/视频"
     if capability in ("image", "video", "audio"):
-        from sqlalchemy import text as _text
+        from sqlalchemy import text as _text, func
         m = db.query(AIModel).filter(
             AIModel.is_enabled == True,
-            AIModel.capabilities.op("JSON_CONTAINS")(
-                AIModel.capabilities, f'"{capability}"'
-            ),
+            func.json_contains(AIModel.capabilities, f'"{capability}"'),
         ).first()
         if m:
             return m