Gitea Actions memory exhaustion through large static matrices

Gitea expanded a workflow's static `strategy.matrix` into its full Cartesian product without a size limit when creating a run, before the fork pull request approval gate applied. A user who can open a pull request from a fork could submit a small workflow file whose matrix expands to a very large number of jobs, consuming server memory and potentially terminating the Gitea process. No runner is required. Static matrices above 256 combinations are now rejected before expansion.