Handle strict filter errors consistently

[Alegr37]

Summary

• Include UndefinedFilter in Context.HandleError so internal errors aren’t swallowed.
• In strictFilters mode, Context.Invoke panics on filter errors; in non-strict mode, it returns the original object.
• Variable.RenderToOutputBuffer converts UndefinedFilter panics to Liquid errors.
• Add tests for strict vs. non-strict filter behavior in context_test.go.