fix: handle arrays properly in FormData

[omarmciver]

Description

This PR fixes an issue with FormData handling when arrays are involved, particularly for multi-file uploads.

The Problem

When an API endpoint accepts an array of files (or any array in FormData), the current implementation incorrectly converts the entire array to a string representation instead of properly appending each item individually to the FormData object.

Example scenario:

// API expects multiple file attachments
const files = [file1, file2, file3];
api.uploadMultiple({ files });

// Current behavior: FormData contains
// "files" => "[object File],[object File],[object File]" ❌

// Expected behavior: FormData should contain
// "files" => file1
// "files" => file2  
// "files" => file3 ✅

The Solution

This fix modifies the FormData content formatter to:

1. Detect when a property value is an array
2. Iterate through each item in the array
3. Properly append each item individually to FormData
4. Preserve the correct handling of Blob/File types vs other data types

Changes Made

• Updated templates/base/http-clients/fetch-http-client.ejs to handle arrays in FormData
• Added test fixture formdata-arrays.json to verify the fix
• All existing tests pass with updated snapshots

Use Cases

This fix is essential for:

• Multi-file upload endpoints
• Batch document processing APIs
• Any FormData field that accepts multiple values
• Services handling attachments (e.g., agreement management systems)

Testing

• ✅ All existing tests pass
• ✅ Snapshots updated to reflect the new array handling
• ✅ Added test fixture for multi-file upload scenario

Backwards Compatibility

This change is fully backwards compatible:

• Single values continue to work as before
• Arrays are now handled correctly instead of being stringified
• No breaking changes to the API