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/linux-sandbox/Cargo.toml

@@ -23,7 +23,6 @@ landlock = { workspace = true }
 libc = { workspace = true }
 seccompiler = { workspace = true }
 serde_json = { workspace = true }
-which = "8.0.0"
 
 [target.'cfg(target_os = "linux")'.dev-dependencies]
 pretty_assertions = { workspace = true }