feat(session): persist previous selections

[rekram1-node]

Summary

• include the previous agent and model in new durable selection events
• keep both fields optional so existing version-1 event logs continue to decode and replay
• prefer durable predecessors when projecting switch messages, with ordered-state fallback for historical events
• update the App replay reducer and generated client/OpenAPI surfaces

This establishes the shared durable contract needed by #41665 before rebasing its Plan plugin work.

Semantics

New selection events snapshot previous from the current Session projection before publication, following the implementation direction agreed for this change.

Validation

• bun test test/session-create.test.ts from packages/core (32 passed)
• focused server-session-v2-reducer.test.ts from packages/app (6 passed)
• bun typecheck from Core, App, Schema, Client, and Protocol
• bun run check:generated from packages/client
• repository pre-push typecheck: 33 tasks passed