Add stable session/delete support

[wayfarer-rus]

Summary

• add stable session/delete capability, request/response models, and JSON-RPC method registration
• wire deletion through AgentSupport, Agent, and Client.deleteSession
• preserve delete capability metadata across stable v1 and unstable v2 conversions and update API baselines

Testing

• ./gradlew jvmTest apiCheck
• focused model serialization, capability conversion, and Ktor integration tests
• full all-platform check was attempted but is blocked on this machine because the iPhone Simulator SDK is unavailable

Tracking

• https://youtrack.jetbrains.com/issue/JUNIE-4145