fix: prevent subscription resolvers from dropping notifications under backpressure (Refs: beans-x0zh)

[hmans]

Summary

• The AgentSessionChanged and ActiveAgentStatuses subscription resolvers blocked on sending each payload to the WebSocket transport via an unbuffered out channel. While blocked, new notifications on the buffer-1 notification channel were silently dropped. This could cause the frontend to miss entire RUNNING status transitions, making the "Agent is working..." spinner disappear while the agent was still active.
• Changed both resolvers to use a latest-value pattern: when a new notification arrives while the resolver is blocked sending, it refreshes the pending payload with the latest session state instead of losing the update. The frontend may skip intermediate states but always sees the most recent one.
• Added TestAgentSessionSubscription_DeliversLatestState to verify correct delivery under rapid notifications.

Test plan

• Run mise test — all Go tests pass
• Manual: start an agent task in Beans UI, verify the "Agent is working..." spinner stays visible throughout
• Manual: trigger rapid agent state changes (e.g. quick tool calls) and verify the spinner doesn't flicker or disappear