Fix multipart array value parsing in HTTP client (#55732)

[joteejotee]

Description

Fixes an issue where the HTTP client would throw InvalidArgumentException: A 'contents' key is required. when sending multipart/form-data requests containing both file attachments and array values (e.g., checkbox selections).

Problem

The parseMultipartBodyFormat method in PendingRequest was unable to handle array values correctly when formatting multipart data. Array values like ['Network Administrator', 'Janitor'] would be passed through as-is instead of being converted to the proper multipart format.

Solution

• Changed map() to flatMap() to properly expand array values
• Array values are now formatted as individual field[] entries for each value
• Maintains backward compatibility with existing name/contents format
• Added comprehensive tests for the new functionality

Changes

• Core Fix: Updated parseMultipartBodyFormat() method in PendingRequest.php
• Tests: Added test cases for array values and mixed file+array scenarios

Testing

• ✅ Array values are correctly formatted as field[] entries
• ✅ Backward compatibility maintained for existing multipart format
• ✅ File uploads with array values work without exceptions
• ✅ All existing tests continue to pass

Example Usage

// This now works without throwing exceptions
Http::attach('attachment', file_get_contents('photo.jpg'), 'photo.jpg')
    ->post('http://example.com/users', [
        'name' => 'Steve',
        'roles' => ['Network Administrator', 'Janitor'], // Array values
    ]);

Fixes #55732