fix: guard zero context length display + add 19 tests for model info

- ModelInfoCard: hide card when effective_context_length <= 0 instead
  of showing 'Context Window: 0 auto-detected'
- Add tests for _normalize_config_for_web model_context_length extraction
- Add tests for _denormalize_config_from_web round-trip (write back,
  remove on zero, upgrade bare string to dict, coerce string input)
- Add tests for CONFIG_SCHEMA ordering (model_context_length after model)
- Add tests for GET /api/model/info endpoint (dict config, bare string,
  empty model, capabilities, graceful error handling)

web/src/components/ModelInfoCard.tsx

@@ -46,7 +46,7 @@ export function ModelInfoCard({ currentModel, refreshKey = 0 }: ModelInfoCardPro
     );
   }
 
-  if (!info || !info.model) return null;
+  if (!info || !info.model || info.effective_context_length <= 0) return null;
 
   const caps = info.capabilities;
   const hasCaps = caps && Object.keys(caps).length > 0;