Add MCP session reconnection with Ping health check

[Nerja]

This PR adds resilience to the MCP toolset by automatically attempting to reconnect when the connection to the MCP server is lost. When ListTools() or CallTool() fails, the client first pings the server to verify the connection is actually dead before attempting a reconnect.

[gemini-code-assist[bot]]

Summary of Changes

Hello @Nerja, I'm Gemini Code Assist[^1]! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the resilience of the MCP toolset by introducing an automatic session reconnection capability. It ensures that any previously established MCP session is validated for health using a Ping mechanism before being returned. If the session is found to be unhealthy, it is gracefully closed, and a new connection is initiated, thereby improving the reliability of interactions with MCP servers and preventing operations on stale connections.

Highlights

• MCP Session Health Check: Implemented a Ping health check for existing MCP sessions within the getSession method to verify their liveness before use.
• Automatic Session Reconnection: Added logic to automatically close an unhealthy or disconnected MCP session (if Ping fails) and trigger the establishment of a new connection.
• Session Reconnection Test: Introduced a new unit test, TestSessionReconnection, to thoroughly validate the new session health check and reconnection mechanism.

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[dpasiukevich]

Thanks for the PR!

I'm thinking -- now we will always be making 2 network requests: ping + list/call_tool.

What do you think about these approaches:

1. Making a detached goroutine which will ping over an interval configured by user (with some default) and refresh the connection. And we'd probably would need to expose Toolset struct (which is now called set) with Close() method so users could clean up this goroutine.
2. Or another option -- handling mcp.ErrConnectionClosed. If list/call_tool return this error, then mcptoolset will refresh the connection. Then no public API changes needed, but we'd still probably would need to call ping before the refresh and do some gradual fallback.

What do you think?

[Nerja]

@dpasiukevich I updated the PR to only ping when there is an error. What do you think of the current approach? I noticed that not all errors are mcp.ErrConnectionClosed (for instance, the Close() call in the tests does not cause mcp.ErrConnectionClosed) so made the compromise to issue a ping when there is any error. If ping succeeds, then we will not create a new connection/session.