Add Queued A2A protocol adapter (A2A over RabbitMQ / Azure Service Bus)

[rockfordlhotka]

Summary

• Adds a new QueuedA2A protocol adapter so agents that communicate via async message brokers (RabbitMQ, Azure Service Bus) can register with the registry and be discovered by clients
• New API surface: POST/GET /a2a/async/agents and GET /a2a/async/agents/{id}
• The QueuedAgentCard extends the A2A agent card with a queueEndpoint object carrying broker connection details (technology, host, port, virtualHost, exchange, taskTopic, responseTopic for RabbitMQ; namespace and entityPath for Azure Service Bus)
• ProtocolType stays A2A (wire format unchanged); TransportType.Amqp and TransportType.AzureServiceBus distinguish broker technologies — no domain layer changes required
• All card fields round-trip faithfully through Endpoint.ProtocolMetadata (JSONB)
• 16 integration tests covering both broker types, 400/401/404 error cases, and full field round-tripping
• Full documentation in docs/protocol-queued-a2a.md and updated README.md

Test plan

• dotnet build — clean (warnings pre-existing)
• dotnet test — 125/125 pass (11 application + 114 API including 16 new QueuedA2A tests)
• Registration with RabbitMQ card returns 201 with registry-assigned ID
• Registration with Azure Service Bus card returns 201
• Missing taskTopic returns 400
• Unauthenticated registration returns 401
• GET /a2a/async/agents/{id} returns card with full queueEndpoint
• HTTP-only A2A agents are excluded from queued endpoint (404)
• List endpoint filters out HTTP A2A agents
• All fields (version, skills with original string IDs, I/O modes, all queue fields) round-trip without loss

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