feat(pi): preserve provider/model fields and sync Pi default provider/model on switch

- Fix stale configRef in useConfig so saves include the latest state.
- Preserve Pi models.json fields (headers, compat, cost, maxTokens,
  thinkingLevelMap, authHeader, modelOverrides, provider name) across edits.
- Add Pi settings.json I/O and update defaultProvider/defaultModel when
  activating a provider so Pi actually picks up the switch.
- Add round-trip and settings serialization tests.

src-tauri/src/kimi_code_io.rs

@@ -476,6 +476,7 @@ api_key = ""
                 official_url: None,
                 managed: false,
                 enabled: true,
+                active: true,
                 raw_other: Value::Null,
             },
         );