Fix listing export custom field data

[Arafat-plugins]

PR Type

What kind of change does this PR introduce?

• Bugfix
• Security fix
• Improvement
• New Feature
• Refactoring (no functional changes, no api changes)
• Build related changes
• Text changes
• Other... Please describe:

Description

How to reproduce the issue or how to test the changes

This fixes the Directorist listing CSV export missing field data when exported listings do not all share the same directory/form field structure.

Previously, the exporter selected one row with the highest column count and used that row as the master CSV header. Fields that existed only on other listings or directories were dropped from the final CSV. This could hide saved listing data such as address, price, date, checkbox, and other custom field values.

This change builds the CSV header from the union of all exported row keys while preserving first-seen order, so fields from every exported listing are included. It also prepares serialized scalar-list field values before CSV output, so checkbox/multi-value custom fields export as readable values instead of raw PHP serialized strings. Complex structured array fields remain serialized to preserve existing CSV import compatibility. CSV generation now uses PHP's native fputcsv() handling for safer escaping of commas, quotes, and multiline content.

1. Create or use listings across directories/forms where one listing has fields that another listing does not have, for example address, pricing, date, and checkbox/custom fields.
2. Go to Directorist -> Tools -> Export and export listings.
3. Open the generated CSV.
4. Verify the CSV header includes field columns from all exported listings, not only the row with the highest column count.
5. Verify saved address, price, date, checkbox, and custom field values are present in their relevant listing rows.
6. Verify checkbox/multi-value scalar fields are exported as readable values instead of raw serialized strings.
7. Verify structured array fields such as social links remain import-compatible.

Local verification performed:

• php -l includes/classes/class-listings-export.php
• vendor\bin\phpcs.bat --standard=phpcs.xml includes/classes/class-listings-export.php
• git diff --check
• Created a temporary local listing with address, price, date, and checkbox fields, exported via Directorist\Listings_Exporter::get_listings_data_as_csv_content(), verified all required columns and values were present, and confirmed checkbox array output was not serialized. The temporary listing and directory were deleted after the test.
• Ran a targeted compatibility check confirming missing mixed-field headers are preserved, checkbox values are readable, formula escaping still works, and structured array values remain serialized for CSV import roundtrip compatibility.

Any linked issues

Fixes # https://team.sovware.com/support/directorist/3270

Screenshot

• N/A

Checklist

• My code follows the WordPress coding standards

[Arafat-plugins]

Closing this PR as requested. The implementation needs to be reworked before resubmission.