Support MCP Events in T3's MCP server #19480

Why

External agents like ChatGPT can start T3 threads through MCP, but once their own turn ends, they aren't notified when those threads finish, fail, or need input.

MCP Events would let external agents subscribe to these changes and automatically resume their conversations, without polling or manual intervention.

How

• Add MCP 2026-07-28 support and expose events/list, events/subscribe, and events/unsubscribe through the existing /mcp endpoint.
• Reuse T3's existing run.updated and runtime-request.updated event streams to publish thread-specific updates, filterable by thread ID.
• Persist subscriptions and deliver verified, signed HTTPS callbacks per the MCP Events spec.

Related: #18490 (MCP protocol negotiation)