Assemble sandbox/approval/network prompts dynamically (#8961)

- Add a single builder for developer permissions messaging that accepts
SandboxPolicy and approval policy. This builder now drives the developer
“permissions” message that’s injected at session start and any time
sandbox/approval settings change.
- Trim EnvironmentContext to only include cwd, writable roots, and
shell; removed sandbox/approval/network duplication and adjusted XML
serialization and tests accordingly.

Follow-up: adding a config value to replace the developer permissions
message for custom sandboxes.

codex-rs/core/src/thread_manager.rs

@@ -450,6 +450,7 @@ mod tests {
             RolloutItem::ResponseItem(items[0].clone()),
             RolloutItem::ResponseItem(items[1].clone()),
             RolloutItem::ResponseItem(items[2].clone()),
+            RolloutItem::ResponseItem(items[3].clone()),
         ];
 
         assert_eq!(