Update V8 Version

Core Workflow

1. Read third_party/v8/README.md and follow its version-bump sequence. Treat that document as the release-process source of truth.
2. Inspect and update the concrete repo surfaces that carry the pin:
   • codex-rs/Cargo.toml
   • codex-rs/Cargo.lock
   • MODULE.bazel
   • third_party/v8/BUILD.bazel
   • third_party/v8/README.md
   • the matching third_party/v8/rusty_v8_<version>.sha256 manifest when the remaining prebuilt inputs change
3. Keep the existing checksum helpers in the loop: