Fix MCP registration ability schemas - #1036

Summary

• Normalize the acf/register-custom-post-type and acf/register-custom-taxonomy input schemas by moving required fields to object-level required arrays.
• Add an items schema to the custom post type supports array.
• Align the supports description with the published array schema.

Validation

• Compared the registration schemas with the existing acf/register-field-group schema pattern.
• Verified the resulting registration schemas against the JSON Schema Draft-07 meta-schema.

Fixes #1035