agents Adds event observability and admin capabilities

Agents manage a lot of state and emit many events that are important to monitor during development and in production.

While working on @fiberplane/agents, we’ve instrumented the Agent class using a mixin (previously a decorator). We’ve realized this should be built into the framework and made easily pluggable.

This PR overlaps with #238 — we should likely merge or coordinate the two.

In addition to what’s in #238, this PR:

Records slightly different set of events (including request and outgoing events like: response and broadcast/websocket send events).
Proposes the following changes to the framework:
1. Admin route inside the worker to expose events stream. This enables embedded interfaces (like Django or Elixir LiveView) to be built on top for debugging or development workflows. This is exposed as an SSE stream and can be accessed via the getAgentEventStream function
2. Added a property on the Agent that accepts an array of EventObservers (which is similar to what is proposed in #238 but allows for multiple event observers).
3. Added a secret method that returns a random string, to make guessing of the uri for events harder, though i don't think this uri should be accessible (this is also only an issue if the agent is exposed to incoming traffic from the internet)
4. Added a listMcpConnections method so connection information is serializable/compatible with RPC