[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/config.md

@@ -148,12 +148,20 @@ Determines when the user should be prompted to approve whether Codex can execute
 approval_policy = "untrusted"
 ```
 
+If you want to be notified whenever a command fails, use "on-failure":
 ```toml
 # If the command fails when run in the sandbox, Codex asks for permission to
 # retry the command outside the sandbox.
 approval_policy = "on-failure"
 ```
 
+If you want the model to run until it decides that it needs to ask you for escalated permissions, use "on-request":
+```toml
+# The model decides when to escalate
+approval_policy = "on-request"
+```
+
+Alternatively, you can have the model run until it is done, and never ask to run a command with escalated permissions:
 ```toml
 # User is never prompted: if the command fails, Codex will automatically try
 # something out. Note the `exec` subcommand always uses this mode.