mcp: make transports open structs

As described in #272, there is really no reason for transports to be
closed structs with constructors, since their state must be established
by a call to Connect. Making them open structs simplifies their APIs,
and means that all transports can be extended in the future: we don't
have to create empty Options structs just for the purpose of future
compatibility.

For now, the related constructors and options structs are simply
deprecated (with go:fix directives where possible). A future CL will
remove them prior to the v1.0.0 release.

For #272

examples/server/hello/main.go

@@ -58,7 +58,7 @@ func main() {
 		log.Printf("MCP handler listening at %s", *httpAddr)
 		http.ListenAndServe(*httpAddr, handler)
 	} else {
-		t := mcp.NewLoggingTransport(mcp.NewStdioTransport(), os.Stderr)
+		t := &mcp.LoggingTransport{Transport: &mcp.StdioTransport{}, Writer: os.Stderr}
 		if err := server.Run(context.Background(), t); err != nil {
 			log.Printf("Server failed: %v", err)
 		}

examples/server/memory/main.go

@@ -137,7 +137,7 @@ func main() {
 		log.Printf("MCP handler listening at %s", *httpAddr)
 		http.ListenAndServe(*httpAddr, handler)
 	} else {
-		t := mcp.NewLoggingTransport(mcp.NewStdioTransport(), os.Stderr)
+		t := &mcp.LoggingTransport{Transport: &mcp.StdioTransport{}, Writer: os.Stderr}
 		if err := server.Run(context.Background(), t); err != nil {
 			log.Printf("Server failed: %v", err)
 		}

examples/server/sequentialthinking/main.go

@@ -536,7 +536,7 @@ func main() {
 			log.Fatal(err)
 		}
 	} else {
-		t := mcp.NewLoggingTransport(mcp.NewStdioTransport(), os.Stderr)
+		t := &mcp.LoggingTransport{Transport: &mcp.StdioTransport{}, Writer: os.Stderr}
 		if err := server.Run(context.Background(), t); err != nil {
 			log.Printf("Server failed: %v", err)
 		}