Align orchestration tracker/streamer tests with current APIs

Update the child tracker and event streamer tests to the tracker's
current shape: `OrchestrationChildTracker::new` takes only the parent
task id, `ChildSignal::Registered` is a unit variant, and discovery
inserts a tracked child before the metadata fetch resolves.

Restructure the tracker tests around a shared `with_tracker` harness so
each case covers a single invariant, and add coverage for the killed-run,
malformed-input, pending-session-link, and seed-row paths.

Co-Authored-By: Oz <oz-agent@warp.dev>

Author: cephalonaut

app/src/ai/blocklist/orchestration_child_tracker.rs

@@ -539,7 +539,6 @@ impl OrchestrationChildTracker {
     pub(crate) fn has_in_flight_fetch(&self, run_id: &str) -> bool {
         self.metadata_fetches.contains(run_id)
     }
-
 }
 
 #[cfg(test)]