fix(jetstream): use ReturnType<typeof setTimeout> for bulk timer

setTimeout returns an opaque Timeout type in newer Deno
releases, so the previous timer: number annotation fails type
checking in CI. Using ReturnType<typeof setTimeout> is portable
across Deno versions and runtimes.

src/jetstream/bulk.ts

@@ -15,7 +15,7 @@ export type BulkDeps = {
 
 type BulkBuffer = {
   records: Record<string, unknown>[];
-  timer: number;
+  timer: ReturnType<typeof setTimeout>;
 };
 
 const bulkBuffers = new Map<string, BulkBuffer>();