feat: add PHP 8.5 and 8.6 compatibility tests and CI matrix entries

[Copilot]

Extends CI coverage and parser compatibility tests to PHP 8.5 (stable since Nov 2025) and PHP 8.6 (in development).

Changes

• CI matrix — adds 8.5 as a required job; adds 8.6 and nightly with continue-on-error: true since both are pre-stable targets
• PHP 8.5 fixture — exercises the new pipe operator (|>), the most impactful new syntax
• PHP 8.6 fixture — exercises readonly properties with default values, which were a fatal error in PHP 8.1–8.5
• SupportedVersionTest — two new version_compare-gated cases verify the AST parser produces correct metrics for each fixture

// php8.5.pipe.php — new |> operator
class Foo {
    public function bar($a) {
        return $a |> strtolower(...) |> trim(...);
    }
}

// php8.6.readonlydefault.php — readonly with default (previously fatal)
class Foo {
    public readonly string $languageCode = 'en';
    public readonly string $countryCode = 'US';
}

Tests follow the existing SupportedVersionTest pattern and are no-ops on PHP versions below the feature threshold.