Extend bundled package discovery and expose its version (#37886)

## What changed

- Recognize executables under `codex-resources/` as part of a package layout by resolving the sibling `bin/` directory, while requiring that directory to exist.
- Add `InstallContext::package_manifest()` to parse the semantic package version from `codex-package.json`.

## Testing

- Extend the package-layout test fixture with a complete manifest and verify that version `1.2.3` is returned.

GitOrigin-RevId: 034aad3e8b865c6c6ecdff47da7c12c969744404

codex-rs/install-context/Cargo.toml

@@ -15,6 +15,9 @@ workspace = true
 [dependencies]
 codex-utils-absolute-path = { workspace = true }
 codex-utils-home-dir = { workspace = true }
+semver = { workspace = true, features = ["serde"] }
+serde = { workspace = true, features = ["derive"] }
+serde_json = { workspace = true }
 
 [dev-dependencies]
 pretty_assertions = { workspace = true }