Add a manager for MCP event streams

[copyberry]

Add a manager for MCP event streams

What changed

• Add McpEventStreamManager to own event stream workers by thread and subscription.
• Wait for activation before completing a start, tag updates with a stream attempt ID, and keep active streams alive independently of their originating runtime.
• Support cancellation and shutdown without depending on output queue capacity, and close streams when MCP access changes or an event server is removed.
• Reconnect reloaded threads to surviving streams so server removal still cancels them.

Testing

Add coverage for activation, replacement attempts, task reloads, cancellation before activation, full output queues, access changes, and shutdown.

[github-actions]

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