Add DeleteStreamsForSessionAsync to ISseEventStreamStore

Is your feature request related to a problem? Please describe.

When an MCP session ends, the SSE event streams stored by ISseEventStreamStore for that session are not proactively cleaned up. They persist until cache expiration removes them. This wastes storage, especially in high-throughput scenarios with many short-lived sessions.

Describe the solution you'd like

Add a DeleteStreamsForSessionAsync(string sessionId, CancellationToken) method to ISseEventStreamStore that deletes all stored streams and their associated events for a given session.

Describe alternatives you've considered

An alternative is to rely solely on cache expiration to keep resource consumption low. Since it is relatively easy to catch session terminations (with RunSessionHandler), it seems reasonable to provide a means to delete the streams for this session since they are known to be no longer needed.

Additional context

Start at ISseEventStreamStore and trace its implementations and callers, especially session termination through RunSessionHandler. Add the requested asynchronous deletion operation and verify that all streams and associated events for the given session are removed, with cancellation supported.