Preserve multiline settings when saving resource hint fields

[mehul0810]

Finding

The React settings save flow sends JSON payloads through perform_save_settings. The server currently cleans the decoded payload recursively before it looks up field definitions, so textarea fields can lose their multiline shape before field-specific sanitization runs.

Impact

Resource hint fields such as DNS Prefetch and Preconnect are intended to support one value per line. If newlines are collapsed before the final sanitizer/split step, saved settings can be merged into a single invalid value and the frontend output becomes unreliable.

Suggested implementation

• Decode the JSON payload, then look up each field definition before choosing the sanitizer.
• Use sanitize_textarea_field() for textarea fields before converting newline-separated values into arrays.
• Keep existing perform_settings option keys and saved value shape compatible.
• Keep masked secret handling intact.

Acceptance criteria

• Saving multiple DNS Prefetch values preserves one entry per line.
• Saving multiple Preconnect values preserves one entry per line.
• Existing single-line values continue to save as expected.
• Masked secret placeholders still preserve the existing stored secret.
• Add focused PHPUnit or Playwright coverage for multiline settings saves.

[mehul0810]

Completed in PR #98 against release/1.6.0. Multiline resource hint settings are preserved through the React settings save path.