Implement the HTTP client adapter that delegates document conversion
to a remote Docling Serve instance via its /v1/convert/file endpoint.
Switchable via CONVERSION_ENGINE=remote env var. Includes health check,
API key auth, response parsing, and 30 new tests covering parsing,
type mapping, HTTP calls, and DI wiring.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
