internal/mcp: dynamically determine the mcp server based on URL

Based on my understanding, the getServer function is used to determine
which MCP server should be serving the session.

User might want to dynamically determine which MCP server to serve when
a new request comes in. In mcp/examples, two MCP servers are created
serving at the same port but under different URL "/greeter1",
"/greeter2". The MCP client will reach out to different URLs for
different MCP servers.

As a TODO, if the getServer() returns nil, the program will crash when
handing MCP request.

Change-Id: I4431266666e9ea28118e827a94eb1359b1fb3a81
Reviewed-on: https://go-review.googlesource.com/c/tools/+/668535
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Hongxiang Jiang <hxjiang@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

examples/hello/main.go

@@ -33,7 +33,7 @@ func main() {
 	server.AddTools(mcp.MakeTool("greet", "say hi", SayHi))
 
 	if *httpAddr != "" {
-		handler := mcp.NewSSEHandler(func() *mcp.Server {
+		handler := mcp.NewSSEHandler(func(*http.Request) *mcp.Server {
 			return server
 		})
 		http.ListenAndServe(*httpAddr, handler)