fix(acp): add edit field to native-steer fallback test event (#8022)

🤖
## Summary
The Rust test build on `main` is broken. Two PRs each passed on their
own, but together they do not compile: #4741 added a required `edit`
field to queued relay events, and #7568 (merged later) added a test that
builds a queued event without that field. Every Rust CI job (Unit Tests,
Rust Lint, Windows Rust) now fails on `main` and on any PR that rebases
onto it.

This adds `edit: None` to that one test event. `None` means "not an edit
of an earlier message", which is what the test intends. No production
code changes.

### Related issue
Breakage introduced by #7568 on top of #4741. Searched open PRs for an
existing fix: none found.

### Testing
- `cargo clippy -p buzz-acp --all-targets -- -D warnings` passes; `cargo
fmt --check` passes.
- CI on this PR is the authoritative test run.

Signed-off-by: Logan Johnson <loganj@squareup.com>
Co-authored-by: Larry <627498bd4bd1f281a16431e3c6cce3b5c25b6692798c78672298aefbf2f8f8b5@buzz.block.builderlab.xyz>

crates/buzz-acp/src/lib.rs

@@ -4602,6 +4602,7 @@ mod try_native_steer_fallback_log_tests {
                 event: event.clone(),
                 received_at: std::time::Instant::now(),
                 prompt_tag: "mention".into(),
+                edit: None,
             }),
             "queued event must be accepted before the steer attempt"
         );