mcp: negotiate protocol version (#112)

Negotiate the protocol version properly, and set the header on HTTP
transports.

We follow the logic of the Typescript SDK.

- On initialization, the client sends the latest version it can support,
and accepts the version that the server returns if the client supports
it. If not, the connection fails.

- The server accepts the client's version unless it doesn't support it,
in which case it replies with its latest version.

Fixes #103.

mcp/server.go

@@ -647,10 +647,11 @@ func (ss *ServerSession) initialize(ctx context.Context, params *InitializeParam
 		ss.mu.Unlock()
 	}()
 
-	version := "2025-03-26" // preferred version
-	switch v := params.ProtocolVersion; v {
-	case "2024-11-05", "2025-03-26":
-		version = v
+	// If we support the client's version, reply with it. Otherwise, reply with our
+	// latest version.
+	version := params.ProtocolVersion
+	if !slices.Contains(supportedProtocolVersions, params.ProtocolVersion) {
+		version = latestProtocolVersion
 	}
 
 	return &InitializeResult{