Enhance session start/end hook

[rcxwhiz]

Enhancement

At the moment the FastMCP lifespan parameter does not contain any info about the client connection that is beginning/ending (specifically the MCP session ID). This would be useful for remote MCP servers where you want to take some action when a client connects/disconnects. As far as I understand these are the best workarounds right now:

• Use access_token() in the lifespan to get the identity of an authenticated user (does not differentiate between sessions)
• Attach some HTTP middleware to the application listening for connection attempts, harvesting MCP session ID from the response
• Attach some MCP middleware with the on_message() hook to track the first MCP message sent for a session

I believe it would be helpful to add access to the MCP session ID in the lifespan handler.

[rcxwhiz]

Just now seeing that this may be covered by #1546

[Maxi91f]

PR #2359 might partially solve the issue, at least for connection