[approval_policy] Add OnRequest approval_policy (#1865)

## Summary
A split-up PR of #1763 , stacked on top of a tools refactor #1858 to
make the change clearer. From the previous summary:

> Let's try something new: tell the model about the sandbox, and let it
decide when it will need to break the sandbox. Some local testing
suggests that it works pretty well with zero iteration on the prompt!

## Testing
- [x] Added unit tests
- [x] Tested locally and it appears to work smoothly!

codex-rs/core/src/protocol.rs

@@ -150,6 +150,9 @@ pub enum AskForApproval {
     /// the user to approve execution without a sandbox.
     OnFailure,
 
+    /// The model decides when to ask the user for approval.
+    OnRequest,
+
     /// Never ask the user to approve commands. Failures are immediately returned
     /// to the model, and never escalated to the user for approval.
     Never,