Add ContentTypeAwarePayload and ContentTypeAwareEncoder

[veewee]

Summary

• Adds ContentTypeAwarePayload<T> generic DTO that pairs any payload with a content-type string
• Adds ContentTypeAwareEncoder<T> decorator that wraps any existing encoder to set the Content-Type header from the payload metadata
• Composable: works with any encoder (Raw, Stream, ResourceStream, etc.) without modifying existing code

Usage

$encoder = new ContentTypeAwareEncoder(
    ResourceStreamEncoder::createWithAutodiscoveredPsrFactories()
);

// In a request handler:
$payload = new ContentTypeAwarePayload('application/pdf', $resourceStream);

Test plan

• Unit test: wraps RawEncoder, verifies body content and Content-Type header
• Unit test: Content-Type from payload overrides pre-existing header
• Psalm passes
• PHP-CS-Fixer passes
• 100% code coverage maintained