app-server: propagate nested experimental gating for AskForApproval::Reject (#14191)
## Summary
This change makes `AskForApproval::Reject` gate correctly anywhere it
appears inside otherwise-stable app-server protocol types.
Previously, experimental gating for `approval_policy: Reject` was
handled with request-specific logic in `ClientRequest` detection. That
covered a few request params types, but it did not generalize to other
nested uses such as `ProfileV2`, `Config`, `ConfigReadResponse`, or
`ConfigRequirements`.
This PR replaces that ad hoc handling with a generic nested experimental
propagation mechanism.
## Testing
seeing this when run app-server-test-client without experimental api
enabled:
```
initialize response: InitializeResponse { user_agent: "codex-toy-app-server/0.0.0 (Mac OS 26.3.1; arm64) vscode/2.4.36 (codex-toy-app-server; 0.0.0)" }
> {
> "id": "50244f6a-270a-425d-ace0-e9e98205bde7",
> "method": "thread/start",
> "params": {
> "approvalPolicy": {
> "reject": {
> "mcp_elicitations": false,
> "request_permissions": true,
> "rules": false,
> "sandbox_approval": true
> }
> },
> "baseInstructions": null,
> "config": null,
> "cwd": null,
> "developerInstructions": null,
> "dynamicTools": null,
> "ephemeral": null,
> "experimentalRawEvents": false,
> "mockExperimentalField": null,
> "model": null,
> "modelProvider": null,
> "persistExtendedHistory": false,
> "personality": null,
> "sandbox": null,
> "serviceName": null
> }
> }
< {
< "error": {
< "code": -32600,
< "message": "askForApproval.reject requires experimentalApi capability"
< },
< "id": "50244f6a-270a-425d-ace0-e9e98205bde7"
< }
[verified] thread/start rejected approvalPolicy=Reject without experimentalApi
```
---------
Co-authored-by: celia-oai <celia@openai.com>
d5694529ca