| Fix the following issue: | |
| ## Problem Statement | |
| 我在用 create-market 表单建分类市场（categorical），发现校验有点漏。比如我两个 outcome 填了一模一样的答案，它居然能过；还有我把某个 outcome 留空，它也不一定报错，就这么放过去了。感觉像是分类答案那块的校验没盖全。最好能让重复的、空的答案都老老实实报出来，别让我糊里糊涂就提交了。 | |
| ## Expected outcomes | |
| - Blank categorical market outcomes are reported with the user-facing error message `Answer cannot be blank`. | |
| - Categorical market outcomes with the same case-sensitive value are reported with the user-facing error message `Category must be unique` for each submitted answer involved in the duplication. | |
| - The categorical-outcome error output lets callers associate each reported error with the submitted answer that caused it, without reordering or losing the submitted outcome positions. | |
| - Missing or empty categorical-outcome input is treated as having no categorical outcome errors. | |
| - The create-market form’s categorical step runs categorical-outcome validation whenever categorical outcomes are present in form state. | |
| ## Implementation notes | |
| The exact module organization, helper functions, and placement of validation logic are up to the implementer. Preserve the existing create-market form validation style and public user-facing messages, but avoid relying on any particular internal file layout or refactor path. |