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/tui/src/additional_dirs.rs

@@ -13,7 +13,9 @@ pub fn add_dir_warning_message(
     }
 
     match sandbox_policy {
-        SandboxPolicy::WorkspaceWrite { .. } | SandboxPolicy::DangerFullAccess => None,
+        SandboxPolicy::WorkspaceWrite { .. }
+        | SandboxPolicy::DangerFullAccess
+        | SandboxPolicy::ExternalSandbox { .. } => None,
         SandboxPolicy::ReadOnly => Some(format_warning(additional_dirs)),
     }
 }
@@ -32,6 +34,7 @@ fn format_warning(additional_dirs: &[PathBuf]) -> String {
 #[cfg(test)]
 mod tests {
     use super::add_dir_warning_message;
+    use codex_core::protocol::NetworkAccess;
     use codex_core::protocol::SandboxPolicy;
     use pretty_assertions::assert_eq;
     use std::path::PathBuf;
@@ -50,6 +53,15 @@ mod tests {
         assert_eq!(add_dir_warning_message(&dirs, &sandbox), None);
     }
 
+    #[test]
+    fn returns_none_for_external_sandbox() {
+        let sandbox = SandboxPolicy::ExternalSandbox {
+            network_access: NetworkAccess::Enabled,
+        };
+        let dirs = vec![PathBuf::from("/tmp/example")];
+        assert_eq!(add_dir_warning_message(&dirs, &sandbox), None);
+    }
+
     #[test]
     fn warns_for_read_only() {
         let sandbox = SandboxPolicy::ReadOnly;