internal/mcp: simplify content

Now that the protocol package is merged into the mcp package, clean up
the higher level 'Content' union types, in favor of types constructors.

Change-Id: I37bedb0ff0a296eef799ba65c37ca7f94ee1dbbe
Reviewed-on: https://go-review.googlesource.com/c/tools/+/672415
Reviewed-by: Jonathan Amsterdam <jba@google.com>
TryBot-Bypass: Jonathan Amsterdam <jba@google.com>

mcp/sse_example_test.go

@@ -20,7 +20,7 @@ type AddParams struct {
 
 func Add(ctx context.Context, cc *mcp.ServerConnection, params *AddParams) ([]mcp.Content, error) {
 	return []mcp.Content{
-		mcp.TextContent{Text: fmt.Sprintf("%d", params.X+params.Y)},
+		mcp.NewTextContent(fmt.Sprintf("%d", params.X+params.Y)),
 	}, nil
 }