fix(data-drains): lazy-init S3 endpoint DNS check

Creating the check eagerly in openSession could yield an unhandled
rejection if the source produced no chunks (deliver never ran). Defer
construction to the first deliver call so the promise is always awaited.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: waleedlatif1

apps/sim/lib/data-drains/destinations/s3.ts

@@ -188,9 +188,12 @@ export const s3Destination: DrainDestination<S3DestinationConfig, S3DestinationC
     // pay the lookup once. The SDK creates its own connections, so we can't
     // pin the IP — but doing the check before any S3 call still rejects
     // hostnames that resolve to internal targets at the start of the run.
-    const endpointCheck = assertEndpointIsPublic(config.endpoint)
+    // Lazy-init avoids an unhandled rejection if the source yields no chunks
+    // and `deliver` never runs (e.g., a drain with nothing new to export).
+    let endpointCheck: Promise<void> | null = null
     return {
       async deliver({ body, contentType, metadata, signal }) {
+        if (endpointCheck === null) endpointCheck = assertEndpointIsPublic(config.endpoint)
         await endpointCheck
         const key = buildKey(config, metadata)
         await withS3ErrorContext('put-object', () =>