## Why
Concurrent Guardian samples can finish out of order, allowing an older sample to
replace the thread's newer in-memory security risk score.
## What changed
- Record when each `SecurityRiskScore` sample starts while preserving compatibility
with records that have no timestamp.
- Add `ExtensionData::insert_if` so checking and replacing a typed attachment happen
under the same lock.
- Update Guardian V2 to replace the thread's risk score only when the completed sample
is newer than the stored one.
## Testing
- Verify concurrent conditional inserts retain the newest value.
- Verify Guardian-generated risk scores include a sampling timestamp.
GitOrigin-RevId: 6543a4e5497da4c623331050789e91613f931c4b
## 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`
