fix(FN-2117): log swallowed engine runtime errors

- Add structured warning logs in silent catch paths for IPC worker shutdown, plugin unregistration cleanup, child runtime metrics polling, and merger build rollback reset
- Improve merger rollback warning context to make build-verification reset and retry failures easier to diagnose
- Add regression tests covering each new warning path to ensure errors are surfaced without changing existing control flow
- Add StepSessionExecutor test coverage for cherry-pick abort logging when conflict cleanup fails

packages/engine/src/runtimes/child-process-runtime.ts

@@ -450,8 +450,9 @@ export class ChildProcessRuntime
         .then((metrics: unknown) => {
           this.lastMetrics = metrics as RuntimeMetrics;
         })
-        .catch(() => {
-          // Ignore errors, use cached value
+        .catch((err: unknown) => {
+          const msg = err instanceof Error ? err.message : String(err);
+          runtimeLog.warn(`GET_METRICS IPC query failed, using cached value: ${msg}`);
         });
     }