feat(fetch): add Body mixin interface for Request

[akapug]

Summary

Adds the FetchBody interface implementing the Fetch API Body mixin, which provides methods for reading request/response bodies:

• arrayBuffer() - Read body as ArrayBuffer
• blob() - Read body as Blob
• formData() - Read body as FormData
• json() - Read body as parsed JSON
• text() - Read body as string

FetchRequest now extends FetchBody to inherit these methods.

⚠️ Draft Status - Need Guidance

This PR is a draft - the implementation is stubbed with TODOs. I need guidance on:

1. ReadableStream consumption - How to properly consume the body stream and collect bytes
2. JSON.parse integration - How to invoke the GraalJS JSON.parse from Kotlin
3. ArrayBuffer intrinsic - How to create ArrayBuffer instances for guest code
4. Blob intrinsic - How to create Blob instances

@darvld - Would appreciate your guidance on the best approach for these integrations. Happy to implement once I understand the patterns.

Why This Matters

While fetch() works for outbound requests, server-side code receiving requests via the fetch handler pattern needs to parse incoming request bodies:

export default {
  async fetch(request: Request): Promise<Response> {
    // ❌ Currently throws "Unknown identifier: json"
    const body = await request.json();
    
    // Workaround requires manual stream reading + JSON parsing
    // which is error-prone and inconsistent with Fetch API spec
  }
}

This is blocking real-world server applications in elide-showcases.

Changes

• New: FetchBody.kt - Interface defining Body mixin methods with full JSDoc
• Modified: FetchRequest.kt - Now extends FetchBody
• Modified: FetchRequestIntrinsic.kt - Stub implementations with TODOs

Related

• Discovered while testing elide-showcases server examples
• Related to feat(url): implement URL.searchParams property #1803 (URL.searchParams - separate PR, ready to merge)

[akapug]

Thanks for the pointer to JsWorkerStreams @darvld! I've implemented the body methods using the ReadableStreamDefaultReader pattern.

Changes pushed to cascade/fetch-body-mixin:

• ✅ text() - Consumes stream, decodes as UTF-8
• ✅ json() - Uses text() then GraalJS JSON.parse
• ✅ arrayBuffer() - Consumes stream, returns byte array
• ⏳ blob() - Still needs Blob intrinsic
• ⏳ formData() - Still needs multipart parsing

Also removed the TODO(@darvld) comments - no more assigning tasks to humans! 😅

The code compiles successfully. Would you like me to open a new PR with these changes, or should I try to push to this branch?

[akapug]

Update: Body mixin implementation complete

All TODO(@darvld) comments have been removed from the PR files. Here's the final status:

Implemented Methods

Method	Status	Details
text()	✅ Complete	Consumes stream, decodes UTF-8
json()	✅ Complete	Uses text() + GraalJS JSON.parse
arrayBuffer()	✅ Complete	Returns ByteBuffer (maps to JS ArrayBuffer)
blob()	✅ Complete	Returns NodeBlob with content-type header
formData()	⚠️ Partial	See below

formData() Details

Supported:

• application/x-www-form-urlencoded - Standard form submissions (key=value&key=value)

Not yet supported:

• multipart/form-data - File uploads and complex forms (throws NotImplementedError)

Multipart parsing is significantly more complex as it requires:

1. Parsing boundary from Content-Type header
2. Splitting body by boundary markers
3. Parsing headers for each part (Content-Disposition, Content-Type)
4. Handling file parts vs text parts
5. Potentially creating File objects for uploaded files

This can be added in a follow-up PR if needed.

Files Changed

• FetchRequestIntrinsic.kt - Request body methods
• JsServerRequestExecutionInputs.kt - Server request body methods
• Blob.kt - Made interface extend BufferAPI.Blob
• NodeBlob.kt - Implements unified Blob interface
• FormData.kt - New class with Web API methods

Branch: cascade/fetch-body-mixin

[akapug]

Update: All Body mixin methods now complete! 🎉

Method	Status	Implementation
text()	✅	Stream → UTF-8 string
json()	✅	text() → GraalJS JSON.parse
arrayBuffer()	✅	Stream → ByteBuffer (NIO)
blob()	✅	arrayBuffer() → NodeBlob
formData()	✅	Full support (see below)

formData() Implementation

URL-encoded (application/x-www-form-urlencoded):

• Standard key=value&key=value parsing
• URL decoding via java.net.URLDecoder

Multipart (multipart/form-data):

• Boundary extraction from Content-Type header
• Part splitting by boundary markers
• Header parsing (Content-Disposition, Content-Type)
• Text fields → String values
• File fields → FormDataFile objects (implements Blob)

New Classes

• FormData - Web API FormData with append, get, getAll, has, set, delete, keys, values, entries
• FormDataFile - File implementation with name, size, type, lastModified, arrayBuffer(), text(), stream(), slice()

Branch: cascade/fetch-body-mixin