fix(did-you-mean): include post_title in phrase suggester

[faisalahammad]

Summary

Did You Mean only used post_content.shingle, so suggestions missed keywords that live in post_title — like WooCommerce product names. This fix adds an ep_did_you_mean field that combines post_title and post_content via copy_to, and switches the phrase suggester to use that field.

Fixes #3554

Changes

includes/classes/Feature/DidYouMean/DidYouMean.php

Before:

$mapping['mappings']['properties']['post_content']['fields'] = [
    'shingle' => [
        'type'     => 'text',
        'analyzer' => 'trigram',
    ],
];

$search_analyzer = [
    'phrase' => [
        'field'            => 'post_content.shingle',
        'max_errors'       => 2,
        'direct_generator' => [
            [
                'field' => 'post_content.shingle',
            ],
        ],
    ],
];

After:

foreach ( [ 'post_content', 'post_title' ] as $source_field ) {
    $existing_copy_to = $mapping_properties[ $source_field ]['copy_to'] ?? [];
    $existing_copy_to = is_array( $existing_copy_to ) ? $existing_copy_to : [ $existing_copy_to ];

    if ( ! in_array( 'ep_did_you_mean', $existing_copy_to, true ) ) {
        $existing_copy_to[] = 'ep_did_you_mean';
    }

    $mapping_properties[ $source_field ]['copy_to'] = $existing_copy_to;
}

$mapping_properties['ep_did_you_mean'] = [
    'type'     => 'text',
    'analyzer' => 'trigram',
];

$search_analyzer = [
    'phrase' => [
        'field'            => 'ep_did_you_mean',
        'max_errors'       => 2,
        'direct_generator' => [
            [
                'field' => 'ep_did_you_mean',
            ],
        ],
    ],
];

Why: The suggester now has access to the same keywords the search query uses, so misspelled title terms like shir can suggest shirt.

tests/php/features/TestDidYouMean.php

Before: mapping tests only checked post_content.shingle, and there was no test for title-only suggestions.

After: mapping tests now assert the ep_did_you_mean field and copy_to mappings exist, and two new tests verify suggestions from post_title.

Why: Prevents regression and verifies the reported scenario.

Testing

Test 1: Title-only suggestion

1. Create a post with title Sleek Cotton Shirt and empty content.
2. Sync the index and search for shir.
   Result: suggested term is shirt.

Test 2: Existing content suggestions still work

1. Create a post with content Test post.
2. Search for teet.
   Result: suggested term is test.

Automated:

EP_HOST=http://127.0.0.1:8890/ vendor/bin/phpunit --filter TestDidYouMean

All 19 tests pass.