all: remove ServerRequest[T] for concrete T (#342)

Replace all occurrences of ServerRequest[*CallToolParams] and other
concrete instantiations with CallToolRequest and the like.

Make the XXXRequest types aliases, to preserve the convenience of
generics for the internal machinery (see shared.go, for example.)

I will expand the aliases in a followup PR.

examples/server/completion/main.go

@@ -16,7 +16,7 @@ import (
 // a CompletionHandler to an MCP Server's options.
 func main() {
 	// Define your custom CompletionHandler logic.
-	myCompletionHandler := func(_ context.Context, req *mcp.ServerRequest[*mcp.CompleteParams]) (*mcp.CompleteResult, error) {
+	myCompletionHandler := func(_ context.Context, req *mcp.CompleteRequest) (*mcp.CompleteResult, error) {
 		// In a real application, you'd implement actual completion logic here.
 		// For this example, we return a fixed set of suggestions.
 		var suggestions []string