feat: introduce ExternalSandbox policy (#8290)

## Description

Introduced `ExternalSandbox` policy to cover use case when sandbox
defined by outside environment, effectively it translates to
`SandboxMode#DangerFullAccess` for file system (since sandbox configured
on container level) and configurable `network_access` (either Restricted
or Enabled by outside environment).

as example you can configure `ExternalSandbox` policy as part of
`sendUserTurn` v1 app_server API:

```
 {
            "conversationId": <id>,
            "cwd": <cwd>,
            "approvalPolicy": "never",
            "sandboxPolicy": {
                  "type": ""external-sandbox",
                  "network_access": "enabled"/"restricted"
            },
            "model": <model>,
            "effort": <effort>,
            ....
        }
```

codex-rs/core/src/command_safety/is_dangerous_command.rs

@@ -21,8 +21,11 @@ pub fn requires_initial_appoval(
     match policy {
         AskForApproval::Never | AskForApproval::OnFailure => false,
         AskForApproval::OnRequest => {
-            // In DangerFullAccess, only prompt if the command looks dangerous.
-            if matches!(sandbox_policy, SandboxPolicy::DangerFullAccess) {
+            // In DangerFullAccess or ExternalSandbox, only prompt if the command looks dangerous.
+            if matches!(
+                sandbox_policy,
+                SandboxPolicy::DangerFullAccess | SandboxPolicy::ExternalSandbox { .. }
+            ) {
                 return command_might_be_dangerous(command);
             }
 
@@ -83,6 +86,7 @@ fn is_dangerous_to_call_with_exec(command: &[String]) -> bool {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use codex_protocol::protocol::NetworkAccess;
 
     fn vec_str(items: &[&str]) -> Vec<String> {
         items.iter().map(std::string::ToString::to_string).collect()
@@ -150,4 +154,23 @@ mod tests {
     fn rm_f_is_dangerous() {
         assert!(command_might_be_dangerous(&vec_str(&["rm", "-f", "/"])));
     }
+
+    #[test]
+    fn external_sandbox_only_prompts_for_dangerous_commands() {
+        let external_policy = SandboxPolicy::ExternalSandbox {
+            network_access: NetworkAccess::Restricted,
+        };
+        assert!(!requires_initial_appoval(
+            AskForApproval::OnRequest,
+            &external_policy,
+            &vec_str(&["ls"]),
+            SandboxPermissions::UseDefault,
+        ));
+        assert!(requires_initial_appoval(
+            AskForApproval::OnRequest,
+            &external_policy,
+            &vec_str(&["rm", "-rf", "/"]),
+            SandboxPermissions::UseDefault,
+        ));
+    }
 }