Replace deprecated undici formData() method with @fastify/busboy implementation

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This PR addresses the deprecation of undici's formData() method by implementing the recommended alternative using @fastify/busboy.

Background

Undici v5 is EOL as of April 30, 2025, and in v6+ the formData() method was marked as deprecated. The method is still present in v7 but will likely be removed in future releases. Undici recommends using @fastify/busboy for parsing multipart/form-data in server environments.

Changes Made

1. Added @fastify/busboy dependency - The recommended library for form data parsing
2. Created src/http/formDataParser.ts - Custom implementation that handles:
   • multipart/form-data parsing using Busboy
   • application/x-www-form-urlencoded parsing using URLSearchParams
   • Proper error handling for unsupported content types
3. Updated HttpRequest.formData() - Replaced deprecated undici method call
4. Updated HttpResponse.formData() - Replaced deprecated undici method call
5. Removed deprecation ESLint suppressions - No longer needed

API Compatibility

The new implementation maintains 100% backward compatibility with the existing FormData API:

// All existing usage continues to work unchanged
const form = await request.formData();
const name = form.get('name');
const file = form.get('upload') as File;
const allValues = form.getAll('duplicateField');

Test Coverage

All existing tests continue to pass, covering:

• ✅ Basic multipart form field parsing
• ✅ File uploads with filename and content type preservation
• ✅ Duplicate field name handling (getAll() support)
• ✅ URL-encoded form data parsing
• ✅ Error handling for unsupported content types

Example Usage

The new parser handles the same scenarios as before:

// Multipart form data
const multipartForm = await request.formData();
console.log(multipartForm.get('username')); // Works as before

// URL-encoded data
const urlencodedForm = await request.formData(); 
console.log(urlencodedForm.get('email')); // Works as before

// File uploads
const file = multipartForm.get('attachment') as File;
console.log(file.name, file.type); // Preserves metadata

This change ensures Azure Functions is prepared for future undici versions and follows the library's recommended migration path.

Fixes #331.

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