Add example servers for SEP-2053 variant patterns (#4)

Add four new example servers demonstrating different SEP-2053 variant
patterns:

- model-optimized (same tools, different descriptions per LLM family)
- trading (API versioning with deprecation lifecycle)
- research (context budget management)
- Refactor the existing variants-http example into a GitHub-style server
with custom ranking.

Signed-off-by: Kurt Degiorgio <kdegiorgio@bloomberg.net>

go/sdk/examples/server/variants-stdio/main.go

@@ -0,0 +1,47 @@
+// Example: Minimal variant-aware MCP server over stdio. Registers a single
+// variant with one tool to demonstrate the simplest possible setup.
+//
+// Run over stdio:
+//
+//	go run ./examples/server/variants-stdio
+package main
+
+import (
+	"context"
+	"log"
+
+	"github.com/modelcontextprotocol/go-sdk/mcp"
+
+	"github.com/modelcontextprotocol/experimental-ext-variants/go/sdk/variants"
+)
+
+type GreetInput struct {
+	Name string `json:"name" jsonschema:"name to greet"`
+}
+
+type GreetOutput struct {
+	Message string `json:"message"`
+}
+
+func greet(_ context.Context, _ *mcp.CallToolRequest, in GreetInput) (*mcp.CallToolResult, GreetOutput, error) {
+	return nil, GreetOutput{Message: "Hello, " + in.Name + "!"}, nil
+}
+
+func main() {
+	inner := mcp.NewServer(&mcp.Implementation{Name: "greeter", Version: "v1.0.0"}, nil)
+	mcp.AddTool(inner, &mcp.Tool{
+		Name:        "greet",
+		Description: "Greet someone by name",
+	}, greet)
+
+	vs := variants.NewServer(&mcp.Implementation{Name: "greeter", Version: "v1.0.0"}).
+		WithVariant(variants.ServerVariant{
+			ID:          "default",
+			Description: "Default greeting variant.",
+			Status:      variants.Stable,
+		}, inner, 0)
+
+	if err := vs.Run(context.Background(), &mcp.StdioTransport{}); err != nil {
+		log.Fatal(err)
+	}
+}