feat(linux-sandbox): add bwrap support (#9938)

## Summary
This PR introduces a gated Bubblewrap (bwrap) Linux sandbox path. The
curent Linux sandbox path relies on in-process restrictions (including
Landlock). Bubblewrap gives us a more uniform filesystem isolation
model, especially explicit writable roots with the option to make some
directories read-only and granular network controls.

This is behind a feature flag so we can validate behavior safely before
making it the default.

- Added temporary rollout flag:
  - `features.use_linux_sandbox_bwrap`
- Preserved existing default path when the flag is off.
- In Bubblewrap mode:
- Added internal retry without /proc when /proc mount is not permitted
by the host/container.

codex-rs/app-server/src/codex_message_processor.rs

@@ -1484,6 +1484,7 @@ impl CodexMessageProcessor {
         let outgoing = self.outgoing.clone();
         let req_id = request_id;
         let sandbox_cwd = self.config.cwd.clone();
+        let use_linux_sandbox_bwrap = self.config.features.enabled(Feature::UseLinuxSandboxBwrap);
 
         tokio::spawn(async move {
             match codex_core::exec::process_exec_tool_call(
@@ -1491,6 +1492,7 @@ impl CodexMessageProcessor {
                 &effective_policy,
                 sandbox_cwd.as_path(),
                 &codex_linux_sandbox_exe,
+                use_linux_sandbox_bwrap,
                 None,
             )
             .await