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 uniquefor 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.