Enable V8 sandboxing for source-built builds (#21146)

## Summary

This is the first PR in the V8 in-process sandboxing rollout.

It adds the build-system and Rust feature plumbing needed to support
sandboxed V8 builds, then enables sandboxing by default for the
source-built Bazel V8 path that we control directly. It deliberately
keeps the published `rusty_v8` artifact workflows on their current
non-sandboxed contract so this PR can land and ship independently before
we change any released artifacts.

## Rollout plan

- [x] **PR 1: land sandbox plumbing and default source-built Bazel V8 to
sandboxed mode**

- [ ] **PR 2: publish sandbox-enabled release artifacts and add
compatibility validation**
- Produce sandboxed artifact pairs for every released Cargo target that
does not already use the source-built Bazel path.
- Add CI coverage that consumes those sandboxed artifacts and verifies:
    - `codex-v8-poc` reports sandbox enabled
    - `codex-code-mode` builds/tests against the sandboxed path

- [ ] **PR 3: switch release consumers to sandboxed artifacts by
default**
  - Update released artifact selectors/checksums.
- Enable the Rust `v8_enable_sandbox` feature in the default release
path.
- Make the sandboxed artifact family the normal path for published
builds.

- [ ] **PR 4: remove rollout-only compatibility paths**
- Remove the temporary non-sandbox release compatibility config once the
new default has shipped and baked.
  - Keep the invariant tests permanently.

.github/scripts/verify_cargo_workspace_manifests.py

@@ -25,7 +25,10 @@ TOP_LEVEL_NAME_EXCEPTIONS = {
 UTILITY_NAME_EXCEPTIONS = {
     "path-utils": "codex-utils-path",
 }
-MANIFEST_FEATURE_EXCEPTIONS = {}
+MANIFEST_FEATURE_EXCEPTIONS = {
+    "codex-rs/code-mode/Cargo.toml": {"sandbox": ("v8/v8_enable_sandbox",)},
+    "codex-rs/v8-poc/Cargo.toml": {"sandbox": ("v8/v8_enable_sandbox",)},
+}
 OPTIONAL_DEPENDENCY_EXCEPTIONS = set()
 INTERNAL_DEPENDENCY_FEATURE_EXCEPTIONS = {}