[Task] Prototype A2A Agent Card artifact type (Quick Win)

[carlesarnal]

Parent Feature

Part of #6996 - A2A Agent Card Registry

Description

Create a minimal viable implementation of A2A Agent Card as a new artifact type in Apicurio Registry. This is a quick-win task to establish the foundation for the full Agent Registry feature.

Scope (MVP)

Minimal Implementation

1. New Artifact Type

   • Add AGENT_CARD to artifact type enum
   • Basic JSON validation (valid JSON structure)

2. Simple Storage

   • Store Agent Card as content
   • Use existing artifact versioning

3. Basic Retrieval

   • Retrieve Agent Card by artifact coordinates
   • Return as JSON

Out of Scope (for this task)

• Full A2A schema validation
• /.well-known/agent.json endpoint
• Capability search
• UI changes

Implementation Steps

1. Add AGENT_CARD artifact type to ArtifactType enum
2. Create AgentCardArtifactTypeUtilProvider in schema-util
3. Basic JSON validation in content handler
4. Add to artifact type registry

Acceptance Criteria

• AGENT_CARD type appears in artifact type list
• Agent Card JSON can be registered via REST API
• Invalid JSON is rejected
• Existing tests pass

Effort Estimate

Small - 1-2 days for experienced contributor

Follow-up

This enables the team to start experimenting with Agent Cards while the full implementation (#6996) is designed.