polish: narrow mapSourceToResponse return type

....it will never return a promise.

Author: yaacovCR

src/execution/execute.ts

@@ -1678,16 +1678,15 @@ async function* ensureAsyncIterable(
 function mapSourceToResponse(
   exeContext: ExecutionContext,
   resultOrStream: ExecutionResult | AsyncIterable<unknown>,
-): PromiseOrValue<
+):
   | AsyncGenerator<
       | ExecutionResult
       | InitialIncrementalExecutionResult
       | SubsequentIncrementalExecutionResult,
       void,
       void
     >
-  | ExecutionResult
-> {
+  | ExecutionResult {
   if (!isAsyncIterable(resultOrStream)) {
     return resultOrStream;
   }