fix: support multiselect in form data - #10676

Closes #7365

Summary by cubic

Fixes formDataToJSON so repeated FormData fields (e.g., multi-selects) serialize into flat arrays instead of creating nested arrays. Works for both top-level and nested keys.

Description

• Change: when a key already has an array value, append via concat instead of wrapping again.
• Reason: repeated values with 3+ entries were producing nested arrays like [[a, b], c].
• Context: supports multi-select inputs and nested paths like foo[bar], while preserving existing array-bracket behavior.

Testing

• Added two unit tests in tests/unit/helpers/formDataToJSON.test.js:
  • Repeated top-level key (3 values) stays flat.
  • Repeated nested key foo[bar] (3 values) stays flat.

^{Written for commit 6909861. Summary will update on new commits.}