build: fix ineffective dynamic imports with lazy boundaries (#33690)

Merged via squash.

Prepared head SHA: 38b3c23d6f
Co-authored-by: gumadeiras <5599352+gumadeiras@users.noreply.github.com>
Co-authored-by: gumadeiras <5599352+gumadeiras@users.noreply.github.com>
Reviewed-by: @gumadeiras

src/agents/subagent-registry-runtime.ts

@@ -0,0 +1,7 @@
+export {
+  countActiveDescendantRuns,
+  countPendingDescendantRuns,
+  countPendingDescendantRunsExcludingRun,
+  isSubagentSessionRunActive,
+  resolveRequesterForChildSession,
+} from "./subagent-registry.js";