fix: handle nullable return type from getDownloadUrl

getDownloadUrl() can return string | null, need to handle null case
to fix TypeScript error in build.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: mrjasonroy

src/lib/ai/tools/file-generator/index.ts

@@ -101,7 +101,10 @@ IMPORTANT: After invoking this tool, do NOT include file URLs in your text respo
           // Get presigned URL for private buckets if available
           let downloadUrl: string;
           if (serverFileStorage.getDownloadUrl) {
-            downloadUrl = await serverFileStorage.getDownloadUrl(uploaded.key);
+            const presignedUrl = await serverFileStorage.getDownloadUrl(
+              uploaded.key,
+            );
+            downloadUrl = presignedUrl || uploaded.sourceUrl;
             logger.info("Presigned URL generated:", {
               downloadUrl,
               isPresigned: downloadUrl.includes("X-Amz"),