release: package prebuilt resource binaries (#23759)

## Why

Release packaging should be a staging step once release binaries have
already been built and signed. The Windows release job was downloading
and signing `codex-command-runner.exe` and
`codex-windows-sandbox-setup.exe`, but `scripts/build_codex_package.py`
still rebuilt those helpers while creating the package archives.

That makes the package step slower and, more importantly, risks putting
helper binaries in the archive that were produced after the signing
step. Linux had the same shape for package resources: `bwrap` could be
rebuilt by the package builder instead of being passed in as a prebuilt
release artifact.

This builds on #23752, which fixes `.tar.zst` creation when Windows
runners rely on the repository DotSlash `zstd` wrapper.

## What changed

- Add explicit prebuilt resource inputs to the Codex package builder:
  - `--bwrap-bin`
  - `--codex-command-runner-bin`
  - `--codex-windows-sandbox-setup-bin`
- Make `.github/scripts/build-codex-package-archive.sh` pass resource
binaries from the release output directory when they are already
present.
- Build Linux `bwrap` for app-server release jobs too, so app-server
package creation does not invoke Cargo just to supply the package
resource.
- Keep macOS package creation as a no-Cargo path when `--entrypoint-bin`
is provided, since macOS packages have no resource binaries.
- Add unit coverage showing prebuilt macOS, Linux, and Windows package
inputs result in no source-built binaries.

## Verification

- `python3 -m unittest discover -s scripts/codex_package -p 'test_*.py'`
- `python3 -m py_compile scripts/codex_package/*.py`
- `bash -n .github/scripts/build-codex-package-archive.sh`
- Dry-ran Linux and Windows package builds with fake prebuilt resources
and a nonexistent Cargo path to verify the package builder did not
invoke Cargo.


---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/23759).
* #23760
* __->__ #23759

scripts/codex_package/README.md

@@ -34,12 +34,14 @@ read from `[workspace.package].version` in `codex-rs/Cargo.toml`.
 
 ## Source-built artifacts
 
-Artifacts built from this repository are always built by the package builder in
-one grouped `cargo build` command per package when they are needed:
+Artifacts built from this repository are built by the package builder in one
+grouped `cargo build` command per package when they are needed and no prebuilt
+override was provided:
 
 - all targets: the selected entrypoint, unless `--entrypoint-bin` is provided
-- Linux targets: `bwrap`
-- Windows targets: `codex-command-runner` and `codex-windows-sandbox-setup`
+- Linux targets: `bwrap`, unless `--bwrap-bin` is provided
+- Windows targets: `codex-command-runner` and `codex-windows-sandbox-setup`,
+  unless the corresponding prebuilt helper flags are provided
 
 The default cargo profile is `dev-small` because local iteration should favor
 fast, small builds. Release jobs should pass `--cargo-profile release` and an
@@ -47,6 +49,12 @@ explicit target. Release jobs that already built and signed/notarized the
 entrypoint should pass `--entrypoint-bin` so the package contains that exact
 binary instead of rebuilding it.
 
+Release jobs that already built package resource binaries should also pass the
+corresponding resource flags: `--bwrap-bin` for Linux packages, and
+`--codex-command-runner-bin` plus `--codex-windows-sandbox-setup-bin` for
+Windows packages. This keeps package archive creation as a pure staging step
+after signing instead of rebuilding resources.
+
 `rg` is not built from this repository, so the builder fetches it from the
 DotSlash manifest at `codex-cli/bin/rg`. Downloaded archives are cached under
 `$TMPDIR/codex-package/<target>-rg` and are reused only after the recorded size