Expose generic `SendNotification` on `ServerSession` for custom protocol extensions

Use Case

I am implementing an MCP Server for the Gemini CLI IDE Integration. The specification requires the server to send custom notifications to the client to signal specific events.

Specifically, I need to send:

• ide/diffAccepted
• ide/diffRejected
• ide/contextUpdate

The Problem

Currently, the mcp.ServerSession struct only exposes specific notification helpers (like NotifyProgress and Log). There is no public API to send a generic/custom JSON-RPC notification.

I attempted to use session.Notify(ctx, method, params), but:

1. The conn field on ServerSession is unexported.
2. The mcp.Connection interface's Write method requires *mcp.JSONRPCMessage, which is an unexported struct.

Proposed Solution

Please expose a generic notification method on ServerSession, similar to:

// In mcp/server.go

func (s *ServerSession) SendNotification(ctx context.Context, method string, params any) error {
    return s.conn.Notify(ctx, method, params)
}

Current Workaround

I am currently forced to use unsafe and reflect to access the underlying conn field to call its Notify method, which is fragile for obvious reasons.

Start in mcp/server.go by reading ServerSession and the existing NotifyProgress and Log helpers, then inspect how the internal conn.Notify call is used. Add the public generic notification entry point described in the issue, and confirm that callers can send custom methods such as ide/diffAccepted without unsafe or reflection.