[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/models.rs

@@ -191,6 +191,10 @@ pub struct ShellToolCallParams {
     // The wire format uses `timeout`, which has ambiguous units, so we use
     // `timeout_ms` as the field name so it is clear in code.
     pub timeout_ms: Option<u64>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub with_escalated_permissions: Option<bool>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub justification: Option<String>,
 }
 
 #[derive(Debug, Clone, PartialEq)]
@@ -302,6 +306,8 @@ mod tests {
                 command: vec!["ls".to_string(), "-l".to_string()],
                 workdir: Some("/tmp".to_string()),
                 timeout_ms: Some(1000),
+                with_escalated_permissions: None,
+                justification: None,
             },
             params
         );