fix(handlers): include execution session metadata and skip idempotency key when absent

- Refactored `requestExecutionMetadata` to handle empty `Idempotency-Key` gracefully.
- Added test to validate metadata inclusion of execution session without idempotency key.

sdk/api/handlers/handlers.go

@@ -194,11 +194,11 @@ func requestExecutionMetadata(ctx context.Context) map[string]any {
 			key = strings.TrimSpace(ginCtx.GetHeader("Idempotency-Key"))
 		}
 	}
-	if key == "" {
-		return make(map[string]any)
-	}
 
-	meta := map[string]any{idempotencyKeyMetadataKey: key}
+	meta := make(map[string]any)
+	if key != "" {
+		meta[idempotencyKeyMetadataKey] = key
+	}
 	if pinnedAuthID := pinnedAuthIDFromContext(ctx); pinnedAuthID != "" {
 		meta[coreexecutor.PinnedAuthMetadataKey] = pinnedAuthID
 	}