linux-sandbox: use standalone bundled bwrap (#21255)

**Summary**
- Add `codex-bwrap`, a standalone `bwrap` binary built from the existing
vendored bubblewrap sources.
- Remove the linked vendored bwrap path from `codex-linux-sandbox`;
runtime now prefers system `bwrap` and falls back to bundled
`codex-resources/bwrap`.
- Add bundled SHA-256 verification with missing/all-zero digest as the
dev-mode skip value, then exec the verified file through
`/proc/self/fd`.
- Keep `launcher.rs` focused on choosing and dispatching the preferred
launcher. Bundled lookup, digest verification, and bundled exec now live
in `linux-sandbox/src/bundled_bwrap.rs`; Bazel runfiles lookup lives in
`linux-sandbox/src/bazel_bwrap.rs`; shared argv/fd exec helpers live in
`linux-sandbox/src/exec_util.rs`.
- Teach Bazel tests to surface the Bazel-built `//codex-rs/bwrap:bwrap`
through `CARGO_BIN_EXE_bwrap`; `codex-linux-sandbox` only honors that
fallback in debug Bazel runfiles environments so release/user runtime
lookup stays tied to `codex-resources/bwrap`.
- Allow `codex-exec-server` filesystem helpers to preserve just the
Bazel bwrap/runfiles variables they need in debug Bazel builds, since
those helpers intentionally rebuild a small environment before spawning
`codex-linux-sandbox`.
- Verify the Bazel bwrap target in Linux release CI with a build-only
check. Running `bwrap --version` is too strong for GitHub runners
because bubblewrap still attempts namespace setup there.

**Verification**
- Latest update: `cargo test -p codex-linux-sandbox`
- Latest update: `just fix -p codex-linux-sandbox`
- `cargo check --target x86_64-unknown-linux-gnu -p codex-linux-sandbox`
could not run locally because this macOS machine does not have
`x86_64-linux-gnu-gcc`; GitHub Linux Bazel CI is expected to cover the
Linux-only modules.
- Earlier in this PR: `cargo test -p codex-bwrap`
- Earlier in this PR: `cargo test -p codex-exec-server`
- Earlier in this PR: `cargo check --release -p codex-exec-server`
- Earlier in this PR: `just fix -p codex-linux-sandbox -p
codex-exec-server`
- Earlier in this PR: `bazel test --nobuild
//codex-rs/linux-sandbox:linux-sandbox-all-test
//codex-rs/core:core-all-test
//codex-rs/exec-server:exec-server-file_system-test
//codex-rs/app-server:app-server-all-test` (analysis completed; Bazel
then refuses to run tests under `--nobuild`)
- Earlier in this PR: `bazel build --nobuild //codex-rs/bwrap:bwrap`
- Prior to this update: `just bazel-lock-update`, `just
bazel-lock-check`, and YAML parse check for
`.github/workflows/bazel.yml`


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[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/21255).
* #21257
* #21256
* __->__ #21255

codex-rs/sandboxing/src/bwrap.rs

@@ -18,7 +18,7 @@ const MISSING_BWRAP_WARNING: &str = concat!(
     "Install bubblewrap with your OS package manager. ",
     "See the sandbox prerequisites: ",
     "https://developers.openai.com/codex/concepts/sandboxing#prerequisites. ",
-    "Codex will use the vendored bubblewrap in the meantime.",
+    "Codex will use the bundled bubblewrap in the meantime.",
 );
 const USER_NAMESPACE_WARNING: &str =
     "Codex's Linux sandbox uses bubblewrap and needs access to create user namespaces.";