internal/mcp: add more package documentation, examples

Add more documentation for the mcp package, explaining how to create and
use Clients and Servers. Also add an example of using the SSEHandler to
crete an MCP server.

Change-Id: Ib327826b2cdb1e3418482551c65569ec6f01af33
Reviewed-on: https://go-review.googlesource.com/c/tools/+/668715
Reviewed-by: Jonathan Amsterdam <jba@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Neal Patel <nealpatel@google.com>
Auto-Submit: Robert Findley <rfindley@google.com>

mcp/sse_test.go

@@ -31,11 +31,9 @@ func TestSSEServer(t *testing.T) {
 				}
 			}
 			httpServer := httptest.NewServer(sseHandler)
+			defer httpServer.Close()
 
-			clientTransport, err := NewSSEClientTransport(httpServer.URL)
-			if err != nil {
-				t.Fatal(err)
-			}
+			clientTransport := NewSSEClientTransport(httpServer.URL)
 
 			client := NewClient("testClient", "v1.0.0", nil)
 			sc, err := client.Connect(ctx, clientTransport, nil)