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/app-server-protocol/schema/json/v1/SendUserTurnParams.json

@@ -16,7 +16,7 @@
           "type": "string"
         },
         {
-          "description": "*All* commands are auto‑approved, but they are expected to run inside a sandbox where network access is disabled and writes are confined to a specific set of paths. If the command fails, it will be escalated to the user to approve execution without a sandbox.",
+          "description": "DEPRECATED: *All* commands are auto‑approved, but they are expected to run inside a sandbox where network access is disabled and writes are confined to a specific set of paths. If the command fails, it will be escalated to the user to approve execution without a sandbox. Prefer `OnRequest` for interactive runs or `Never` for non-interactive runs.",
           "enum": [
             "on-failure"
           ],