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/core/src/exec.rs

@@ -140,6 +140,7 @@ pub async fn process_exec_tool_call(
     sandbox_policy: &SandboxPolicy,
     sandbox_cwd: &Path,
     codex_linux_sandbox_exe: &Option<PathBuf>,
+    use_linux_sandbox_bwrap: bool,
     stdout_stream: Option<StdoutStream>,
 ) -> Result<ExecToolCallOutput> {
     let windows_sandbox_level = params.windows_sandbox_level;
@@ -184,14 +185,15 @@ pub async fn process_exec_tool_call(
 
     let manager = SandboxManager::new();
     let exec_env = manager
-        .transform(
+        .transform(crate::sandboxing::SandboxTransformRequest {
             spec,
-            sandbox_policy,
-            sandbox_type,
-            sandbox_cwd,
-            codex_linux_sandbox_exe.as_ref(),
+            policy: sandbox_policy,
+            sandbox: sandbox_type,
+            sandbox_policy_cwd: sandbox_cwd,
+            codex_linux_sandbox_exe: codex_linux_sandbox_exe.as_ref(),
+            use_linux_sandbox_bwrap,
             windows_sandbox_level,
-        )
+        })
         .map_err(CodexErr::from)?;
 
     // Route through the sandboxing module for a single, unified execution path.
@@ -1108,6 +1110,7 @@ mod tests {
             &SandboxPolicy::DangerFullAccess,
             cwd.as_path(),
             &None,
+            false,
             None,
         )
         .await;