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/tool.go

@@ -9,7 +9,6 @@ import (
 	"encoding/json"
 	"slices"
 
-	"github.com/modelcontextprotocol/go-sdk/internal/util"
 	"github.com/modelcontextprotocol/go-sdk/jsonschema"
 )
 
@@ -56,12 +55,12 @@ func NewTool[TReq any](name, description string, handler func(context.Context, *
 		// rather than returned as jsonrpc2 server errors.
 		if err != nil {
 			return &CallToolResult{
-				Content: []WireContent{TextContent{Text: err.Error()}.ToWire()},
+				Content: []Content{NewTextContent(err.Error())},
 				IsError: true,
 			}, nil
 		}
 		res := &CallToolResult{
-			Content: util.Apply(content, Content.ToWire),
+			Content: content,
 		}
 		return res, nil
 	}