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/mcp_connection_manager.rs

@@ -313,6 +313,8 @@ pub struct SandboxState {
     pub sandbox_policy: SandboxPolicy,
     pub codex_linux_sandbox_exe: Option<PathBuf>,
     pub sandbox_cwd: PathBuf,
+    #[serde(default)]
+    pub use_linux_sandbox_bwrap: bool,
 }
 
 /// A thin wrapper around a set of running [`RmcpClient`] instances.