internal/mcp: clean up handling of content

Add support for all content types, and formalize the conversion of
content to and from the wire format (as a discriminated union).

Change-Id: I93678ce98c02176a524d2c82425fb1267ad68bf0
Reviewed-on: https://go-review.googlesource.com/c/tools/+/669135
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Auto-Submit: Robert Findley <rfindley@google.com>

mcp/server_example_test.go

@@ -40,11 +40,11 @@ func ExampleServer() {
 		log.Fatal(err)
 	}
 
-	content, err := serverConnection.CallTool(ctx, "greet", map[string]any{"name": "user"})
+	res, err := serverConnection.CallTool(ctx, "greet", map[string]any{"name": "user"})
 	if err != nil {
 		log.Fatal(err)
 	}
-	fmt.Println(content[0].(mcp.TextContent).Text)
+	fmt.Println(res.Content[0].Text)
 
 	serverConnection.Close()
 	clientConnection.Wait()