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/mod.rs

@@ -167,6 +167,7 @@ pub async fn collect_mcp_snapshot(config: &Config) -> McpListToolsResponseEvent
         sandbox_policy: SandboxPolicy::ReadOnly,
         codex_linux_sandbox_exe: config.codex_linux_sandbox_exe.clone(),
         sandbox_cwd: env::current_dir().unwrap_or_else(|_| PathBuf::from("/")),
+        use_linux_sandbox_bwrap: config.features.enabled(Feature::UseLinuxSandboxBwrap),
     };
 
     mcp_connection_manager