fix(import): preserve tab fields and group subfields during JSON import

[huongcodecb1992]

https://app.asana.com/1/11768486970802/project/17125277541504/task/1214591495083397

[opencode-agent]

Based on my analysis of the changes, I've identified the following issues:

Issues Found

1. Potential Key Collision in convert_fields_for_builder() (MetaBox.php:497-501)

The new code for handling tab fields may cause key collisions:

if ( isset( $field['type'] ) && $field['type'] === 'tab' ) {
    unset( $fields[ $id ] );
    $fields[ $field['_id'] ?? $field['id'] ] = $field;
    continue;
}

If a tab field's _id (or id) happens to be the same as an existing numeric key in the $fields array, this will overwrite the existing entry silently.

2. Missing Group Subfield Handling

The PR title mentions "preserve tab fields and group subfields during JSON import", but the changes only address tab fields. There is no handling added for preserving group subfields. The convert_fields_for_builder method recursively processes group subfields (line 508-510), but the structural keys in Field.php ('_id', 'fields', 'tab') don't specifically address group subfield preservation.

3. Incomplete Tab Array Check in unparse_tabs() (MetaBox.php:189-192)

if ( ! in_array( $field['tab'], $added_tabs, true ) ) {
    $new_fields[ $field['tab'] ] = $tab_field;
    $added_tabs[]                = $field['tab'];
}

The original code incorrectly added $tab_field (the entire field array) instead of $field['tab'] (the tab ID string) to $added_tabs. While this bug is fixed, there's still an edge case: if multiple different tab labels map to the same tab ID, only the first encountered tab field will be added.

4. Potential Null Coalesce Issue in convert_fields_for_builder() (MetaBox.php:499)

$fields[ $field['_id'] ?? $field['id'] ] = $field;

If neither _id nor id exists on a tab field, this will generate a PHP Notice (undefined index) and the field may be lost.

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[rilwis]

Ko cần nữa do đã fix ở đây: wpmetabox/meta-box-builder#186