fix: parse multipart/form-data request bodies in LaravelHttpServer

[Llewdur]

Summary

LaravelHttpServer::handleRequest() only parses
application/x-www-form-urlencoded bodies. A multipart request — the
shape a real <input type="file"> produces when submitted through the
browser — reaches the in-process test server with its body untouched,
and Request::create() is always called with [] for its $files
argument (the // @TODO files...). Concretely: any Laravel app testing
a file upload through visit() gets a 422 from its Form Request,
because the file never arrives.

This PR adds Support\MultipartFormDataParser, a small, dependency-free
parser that turns a raw multipart/form-data body into the
[$parameters, $files] shape Symfony\Component\HttpFoundation\Request::create()
expects, and wires it into handleRequest() alongside the existing
urlencoded branch.

Details

• Text fields are parsed into $parameters, uploaded parts are written
  to a temp file and described in the same $_FILES-style array PHP's
  own SAPI produces (name/type/tmp_name/error/size).
• Bracketed field names (documents[], meta[address]) are supported
  via a small recursive path-setter that mirrors how PHP itself parses
  bracketed form field names — this covers both single- and
  multi-file inputs.
• An empty file input (no file chosen) is reported as
  UPLOAD_ERR_NO_FILE, matching what a real PHP SAPI does, rather than
  being silently dropped or treated as a zero-byte file.
• Symfony\Component\HttpFoundation\FileBag::fixPhpFilesArray()
  already normalizes either the "normal" nested shape or PHP's own
  quirky flattened $_FILES shape, so the parser only needs to build
  the straightforward nested one.

Testing

Unit tests are included at tests/Unit/Support/MultipartFormDataParserTest.php
covering: plain text fields (including multi-line values), a single
file upload, an empty file input, and repeated bracketed fields
(tags[], documents[]) collecting into a list for both text and file
parts.

I wasn't able to run this repo's own suite locally (the sandbox I'm
working from is missing ext-dom, which pestphp/pest itself needs to
boot), so I verified the parser two ways instead, both passing:

1. A standalone script exercising every case in the test file directly
   against MultipartFormDataParser::parse().
2. The same output round-tripped through the real
   Symfony\Component\HttpFoundation\Request::create() +
   FileBag — confirming a bracketed multi-file field converts into a
   list of genuine UploadedFile instances with the correct original
   names and on-disk content, i.e. exactly what handleRequest() does
   with the parser's output.

Happy to adjust anything CI flags once it runs, or to add a visit()-
level Browser test exercising an actual <input type="file"> if that's
preferred over (or in addition to) the unit tests.

Related

We hit this while trying to write a browser test for a document-upload
flow in our own app and found the // @TODO files... — this closes that
gap for us and (hopefully) for anyone else hitting the same 422.