test: demonstrate multipart array encoding bug with empty bracket indices

The SDK's addFormValue uses `key + '[]'` for arrays, producing field
names like `files[][dest_path]` and `files[][file]` instead of
`files[0][dest_path]` and `files[0][file]`.

Without numeric indices, a server cannot:
1. Parse the field names (strconv.Atoi("") fails on empty brackets)
2. Pair file contents with their destination paths for multi-file uploads

These tests use a mock HTTP server with busboy to capture and inspect
the actual multipart field names the SDK produces. Affected endpoints:
  - fs.upload (files array)
  - browsers.loadExtensions (extensions array)

Made-with: Cursor

Author: rgarcia