internal/mcp: change paginateList to a generic helper

This CL simplifies the paginateList function in server.go to use a
generic helper for tools, resources, and prompts.

Change-Id: Ide0d2a90d715374280067e094d8870882e6bddfa
Reviewed-on: https://go-review.googlesource.com/c/tools/+/678055
Auto-Submit: Sam Thanawalla <samthanawalla@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

mcp/shared.go

@@ -271,3 +271,13 @@ type Result interface {
 type emptyResult struct{}
 
 func (*emptyResult) GetMeta() *Meta { panic("should never be called") }
+
+type listParams interface {
+	// Returns a pointer to the param's Cursor field.
+	cursorPtr() *string
+}
+
+type listResult[T any] interface {
+	// Returns a pointer to the param's NextCursor field.
+	nextCursorPtr() *string
+}