feat(jsonld): add resource-level jsonldContext for namespace prefixes

[soyuka]

Summary

• Add jsonldContext parameter to ApiResource and HttpOperation (and all concrete operation classes)
• ContextBuilder merges the resource's jsonldContext into the generated @context output
• YAML and XML extractors updated to support the new field
• Cascade from ApiResource to operations works automatically via WithResourceTrait::copyFrom

Usage

#[ApiResource(
    jsonldContext: ['dct' => 'http://purl.org/dc/terms/'],
)]
class BibliographicResource
{
    #[ApiProperty(iris: ['dct:title'])]
    public ?string $title = null;
}

Produces /contexts/BibliographicResource:

{
    "@vocab": "...",
    "hydra": "...",
    "dct": "http://purl.org/dc/terms/",
    "title": "dct:title"
}

Test plan

• php vendor/bin/phpunit tests/Functional/JsonLd/ContextTest.php — 9 tests pass
• testResourceLevelJsonLdContextAddsNamespacePrefixes covers the new feature end-to-end

Closes #767