Support multipart/form-data request bodies

[danielbowne]

Problem

Emberfall currently supports text and json body types for requests, but there is no way to send multipart/form-data payloads. This limits smoke testing for APIs that accept file uploads via multipart (e.g., resume processors, document ingest endpoints, image upload APIs).

Proposed Solution

Add a multipart option to the body configuration that supports key-value form fields:

tests:
  - url: http://localhost:3000/api/upload
    method: POST
    body:
      multipart:
        fields:
          name: "document.pdf"
          category: "resume"
    expect:
      status: 202

This would set Content-Type: multipart/form-data with the appropriate boundary and encode the fields as form parts.

Use Case

Testing API endpoints that accept multipart form submissions with metadata fields (without file attachments -- see separate issue for file support).