internal/mcp: add pagination for prompts

Change-Id: Ib231427299dbfe807fb80ad8da6ba16d4a8fde03
Reviewed-on: https://go-review.googlesource.com/c/tools/+/676615
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

mcp/server.go

@@ -179,8 +179,17 @@ func (s *Server) Sessions() iter.Seq[*ServerSession] {
 func (s *Server) listPrompts(_ context.Context, _ *ServerSession, params *ListPromptsParams) (*ListPromptsResult, error) {
 	s.mu.Lock()
 	defer s.mu.Unlock()
+	var cursor string
+	if params != nil {
+		cursor = params.Cursor
+	}
+	prompts, nextCursor, err := paginateList(s.prompts, cursor, s.opts.PageSize)
+	if err != nil {
+		return nil, err
+	}
 	res := new(ListPromptsResult)
-	for p := range s.prompts.all() {
+	res.NextCursor = nextCursor
+	for _, p := range prompts {
 		res.Prompts = append(res.Prompts, p.Prompt)
 	}
 	return res, nil