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/protocol/src/protocol.rs

@@ -356,10 +356,12 @@ pub enum AskForApproval {
     #[strum(serialize = "untrusted")]
     UnlessTrusted,
 
-    /// *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.
+    /// 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.
     OnFailure,
 
     /// The model decides when to ask the user for approval.