fix(ai): resolve missing multipart boundary in toMarkdown transform

[Ahmedhajjajofficial]

Description

This PR resolves the 400 {"errors":[{"message":"Request body is not valid multipart/form-data"}]} error when calling the cloudflare.ai.toMarkdown.transform method.

Root Cause

In certain runtime environments (like Node.js), passing a standard FormData instance directly into the internal HTTP fetch wrapper causes a boundary mismatch. The internal request layer hardcodes the content-type: multipart/form-data header without appending the dynamically generated boundary string (e.g., boundary=----WebKitFormBoundary...), causing Cloudflare Gateway to reject the payload immediately.

Solution

Instead of transmitting the raw FormData object, we pipe it into a native Response constructor inside the transform method. This forces the underlying V8/Node engine to serialize the payload and accurately construct a spec-compliant content-type header that includes the official boundary token. We then extract this complete header and apply it explicitly to the final request payload.

Verification

• Resolves the native multipart/form-data parsing failure.
• Fixes GitHub Issue "cloudflare.ai.toMarkdown.transform" does not work #2696.