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.

.github/workflows/rust-ci.yml

@@ -99,6 +99,9 @@ jobs:
       USE_SCCACHE: ${{ startsWith(matrix.runner, 'windows') && 'false' || 'true' }}
       CARGO_INCREMENTAL: "0"
       SCCACHE_CACHE_SIZE: 10G
+      # Keep cargo-based CI independent of system bwrap build deps.
+      # The bwrap FFI path is validated in Bazel workflows.
+      CODEX_BWRAP_ENABLE_FFI: "0"
 
     strategy:
       fail-fast: false
@@ -467,6 +470,9 @@ jobs:
       USE_SCCACHE: ${{ startsWith(matrix.runner, 'windows') && 'false' || 'true' }}
       CARGO_INCREMENTAL: "0"
       SCCACHE_CACHE_SIZE: 10G
+      # Keep cargo-based CI independent of system bwrap build deps.
+      # The bwrap FFI path is validated in Bazel workflows.
+      CODEX_BWRAP_ENABLE_FFI: "0"
 
     strategy:
       fail-fast: false
@@ -502,7 +508,6 @@ jobs:
 
     steps:
       - uses: actions/checkout@v6
-
       # Some integration tests rely on DotSlash being installed.
       # See https://github.com/openai/codex/pull/7617.
       - name: Install DotSlash