chore(core) Deprecate approval_policy: on-failure (#11631)

## Summary
In an effort to start simplifying our sandbox setup, we're announcing
this approval_policy as deprecated. In general, it performs worse than
`on-request`, and we're focusing on making fewer sandbox configurations
perform much better.

## Testing
- [x] Tested locally
- [x] Existing tests pass

codex-rs/docs/codex_mcp_interface.md

@@ -59,7 +59,7 @@ Request `newConversation` params (subset):
 - `model`: string model id (e.g. "o3", "gpt-5.1", "gpt-5.1-codex")
 - `profile`: optional named profile
 - `cwd`: optional working directory
-- `approvalPolicy`: `untrusted` | `on-request` | `on-failure` | `never`
+- `approvalPolicy`: `untrusted` | `on-request` | `on-failure` (deprecated) | `never`
 - `sandbox`: `read-only` | `workspace-write` | `external-sandbox` (honors `networkAccess` restricted/enabled) | `danger-full-access`
 - `config`: map of additional config overrides
 - `baseInstructions`: optional instruction override