fix(v4): append array items individually in __addInputValues for FormData

[AndrewGit25]

Description

Resolves an issue where multi-value elements processed manually by HTMX (e.g., via hx-include, non-form containers, or custom web components whose .value property returns an Array) serialize as a single comma-separated string (fruit=banana,apple) instead of individual keys (fruit=banana&fruit=apple).

Context & Root Cause

• When submitting standard <form> elements, native FormData automatically appends multiple entries for multi-value inputs like <select multiple>.
• However, when HTMX manually collects values via __addInputValues (such as for hx-include or custom elements outside a form), passing an array to FormData.append(name, input.value) invokes Array.prototype.toString(), coercing the array into a comma-separated string.
• This PR updates __addInputValues to detect arrays (Array.isArray(input.value)) and append each item individually, bringing manual value collection into parity with native <form> multi-value serialization.

Changes

• Updated __addInputValues in src/htmx.js to handle Array.isArray(input.value) by iterating over each item and calling formData.append(name, item).
• Added non-null filtering (if (v != null)) so null or undefined values inside arrays or standalone properties are safely skipped rather than stringified to "null".
• Added unit tests in test/tests/unit/__collectFormData.js covering array serialization and null-filtering for both form and non-form elements.

Corresponding issue: N/A

Testing

• Unit test suite for __collectFormData: 37/37 passed.
• Full automated test suite (npm test): All 87 test files (1,670 tests) passed with 0 failures.
• Manual test: Verified with a <custom-select> component returning an array .value, successfully producing repeated query parameters matching native <select multiple> behavior.

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (dev or four-dev)
• This is a bugfix / parity fix for manual form data extraction
• I ran the test suite locally (npm run test) and verified that it succeeded