[PHP] Prepare cautious URL mappings once - #588

Prepares cautious URL mappings once, then shares them with every text processor created by one StructuredDataUrlRewriter.

Before this PR, each processor parsed the same source and target URLs, sorted them, and built the same regular expressions again. A database value may contain many text leaves, so that setup work was repeated for every leaf.

CautiousURLBaseRewriteMapping now owns that setup. A caller which processes several values can reuse it directly:

$url_mapping = new CautiousURLBaseRewriteMapping([
    'https://source.example/media' =>
        'https://destination.example/assets',
]);

foreach ($text_values as $text) {
    $processor = new CautiousURLBaseProcessorInTextWithMixedUnknownEscapeRules(
        $text,
        $url_mapping
    );

    while ($processor->next_url()) {
        $processor->replace_url_base();
    }

    $rewritten_values[] = $processor->get_updated_text();
}

StructuredDataUrlRewriter creates this object in its constructor and passes it to its plain-text and block-markup processors. The mapping is prepared once even when serialized PHP, JSON, or block markup exposes many separate text values.

In the local benchmark, constructing a processor with one mapping fell from 1.75 µs to 0.09 µs. With 64 mappings it fell from 106 µs to 0.09 µs. Rewriting one short matching value fell from 3.23 µs to 1.37 µs.

Testing

The URL-rewriting suite passes with 363 tests and 2,502 assertions. The focused cautious and structured suites pass with 180 tests and 219 assertions. PHPStan and the coding-standard checks pass for the changed classes.