[Lambda] Function URLs: Binary response data support

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**Tell us about your request**

Enable Lambda Function URLs to return binary data (e.g., PDFs, images, Excel files) directly in HTTP responses.

**Which service(s) is this request for?**

AWS Lambda - Lambda Function URLs

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**

Building serverless web applications using Lambda Function URLs that need to serve binary content like PDF reports, Excel exports, and images. Lambda Function URLs cannot return binary data in responses - only text-based content is supported.

This forces unnecessary architectural complexity:
- Additional S3 bucket for temporary file storage
- Pre-signed URL generation and management
- Extra latency (write to S3 → generate URL → redirect client)
- Cleanup logic for temporary files
- Added costs for S3 PUT operations

API Gateway supports binary media types via `binaryMediaTypes` configuration, so this is technically feasible. Function URLs were positioned as a simpler alternative to API Gateway, but this limitation significantly reduces their utility.

**Are you currently working around this issue?**

Yes. We write binary content to an S3 "delivery bucket" and return a pre-signed URL with short expiration. The client follows this URL to download the file. This adds ~200-500ms latency, extra infrastructure, and operational overhead.

**Additional context**

- Base64 encoding works for small images but is impractical for larger files
- The existing 6MB payload limit would still apply - this is about content type, not size
- Common use case: serverless apps generating reports, exports, invoices

No repository files, tests, or implementation entry points are identified in the issue. Start by reviewing the Lambda Function URLs response behavior and API Gateway's binaryMediaTypes reference; done means Function URLs can return PDFs, images, and other binary responses directly while retaining the stated 6MB payload limit.

Written by the indexing model from the issue text.