## What changed
- Add an extension API for spawning host-owned internal sessions and a
`ThreadReadyInput` lifecycle callback that runs after thread registration.
- Add Guardian reviewer session scaffolding that records the parent thread and
effective model, plus the under-development `guardian_ext` feature flag.
- Start internal sessions with fresh history while preserving parent lineage,
shared session controls, and internal-thread visibility rules.
- Scope internal-session prompt cache keys to their source and parent thread,
and expose `guardian` as an internal session source.
## Testing
- Cover internal-session spawning, parent metadata, history isolation, prompt
cache keys, and extension spawner argument forwarding.
GitOrigin-RevId: 682dae80397d62bb36247796b447042e760ca364
## Why
`codex-extension-api` defines contracts shared by extension crates and
their hosts, but it had no direct test suite. Host and feature tests
cover downstream behavior, while regressions in the API crate's own
typed state, registry ordering, and capability adapters could go
unnoticed.
## What
- Add public-surface integration tests for `ExtensionData`, including
concurrent initialization and poison recovery.
- Cover contributor registration order, approval short-circuiting, event
sink retention, no-op response injection, and closure-based agent
spawning.
- Add the test-only dependencies used by the suite.
## Validation
- `just test -p codex-extension-api`
- `just argument-comment-lint -p codex-extension-api`
- `just bazel-lock-check`
