fix: Add multipart form-data support for Laravel HTTP server

[homiedopie]

Problem

• Multipart form-data requests were not fully parsed in the Laravel HTTP server driver.
• As a result, uploaded files and nested multipart fields were not reliably available in the Laravel Request object.

Fix

• Added multipart parsing support using Amp form parser and mapped payloads to Symfony/Laravel-compatible request structures.
• Implemented normalization for nested fields and uploaded files, and converted uploaded parts into Laravel UploadedFile instances.

Test

• URL-encoded form body
• Multipart form with file uploads (from: @gtg-bantonio)
• Multipart form with nested fields (from: @gtg-bantonio)

Verification

• Verified my SPA test cases using form-data

Related

Related #177 - Thanks to @gtg-bantonio
Fixes #1495