Epic: [ACP] Protocol completeness #922
Description
Objective
Fill remaining gaps in stable ACP protocol method implementations.
Scope
- Implement ext_notification handler
- Echo UserMessageChunk during prompt processing
- Add structured logging for skipped content blocks (Audio, ResourceLink)
Acceptance Criteria
- ext_notification does not return method_not_found
- User messages echoed as UserMessageChunk in notifications
- Unsupported content blocks logged with tracing::debug