Fix single Pick values containing commas

[faisalahammad]

Description

Single-value Pick dropdown options containing commas were split during save. This caused valid custom-simple values such as $3,500 - $9,999 to be rejected and saved as empty.

The save path now keeps scalar values intact for single-select fields and keeps comma splitting for multi-select fields. A regression test covers custom-simple dropdown values containing commas across supported storage and Pod types.

Related GitHub issue(s)

Fixes #7374

Testing instructions

1. Create a Pick field with Selection Type set to Single Select.
2. Set format to Drop Down.
3. Add custom options with comma-containing values, such as $3,500 - $9,999.
4. Create or edit a Pod item.
5. Select the comma-containing option and save.
6. Reload the item.
7. Confirm the selected option remains selected and its value and label are unchanged.

Automated regression test:

vendor/bin/codecept run wpunit Pods/Bug/Bug7374Test -vvv

The focused test could not run in this environment because Composer dependencies could not be installed. php -l passed for changed PHP files and git diff --check passed. Composer installation is blocked by existing dependency constraints involving WPGraphQL security advisories, Faker on PHP 8.5, and Polylang minimum stability.

Screenshots / screencast

Not applicable. No UI code changed.

Changelog text for these changes

Bug: Single Pick dropdown values containing commas now save correctly. #7374 (@faisalahammad)

PR checklist

• I have tested my own code to confirm it works as I intended.
• My code follows the WordPress Coding Standards.
• My code follows the WordPress Inline Documentation Standards.
• My code includes automated tests for PHP and/or JS (if applicable).

[faisalahammad]

AI disclosure: this PR was written with Claude Opus 5 assistance, reviewed and tested by me.