Preserve typed property when allOf branches overlap

[cubahno]

Summary

• mergeOpenapiSchemas overwrote properties shared by two allOf branches with whatever the second branch declared, even when that branch only carried an annotation (e.g. example) and no type. The typed side - including its enum/constraints - was silently lost and the field collapsed to struct{} in generated code.
• When a property exists on both sides, keep the typed declaration and only let an untyped override win when the existing entry is also untyped. The original SchemaProxyis preserved so downstream GoLow().GetReference() attribution still works.