refactor(run-engine): stop exporting the internal BlockedRun type

`BlockedRun` is only named inside types.ts, by CompleteResult. The repo's
knip gate rejects unused exports, so drop the export keyword rather than
add a knip.json exception — nothing outside this file needs the name yet.

internal-packages/run-engine/src/engine/waitpointCoordinator/types.ts

@@ -87,7 +87,7 @@ export type CompleteParams = {
 };
 
 /** One run blocked by the completed waitpoint, with the fields the caller's fan-out loop reads. */
-export type BlockedRun = {
+type BlockedRun = {
   taskRunId: string;
   spanIdToComplete: string | null;
   createdAt: Date;