[cubahno]

A query parameter whose schema is a union with two or more non-null variants (id | "null" | null, or a nested anyOf) was generated as an anonymous struct holding a json:"-" field.
That field only works because the parent type's generated MarshalJSON merges it back, and an anonymous struct can't have methods.
So it marshaled to {}, form encoding emitted no pairs, and the parameter was silently missing from the URL: no error, and the server returned an unfiltered result. Path and header parameters share the same code path.

What changes
generateParamsTypes gives a parameter schema a named type, with a generated MarshalJSON, when it would otherwise be an anonymous struct with nameless fields. It reuses the needsMarshaler check that object properties already go through.
The type is named <Operation><Query|Path|Headers>_<Param>, e.g. ListWidgetsQuery_ParentID, the same pattern used for parameters with additionalProperties.
The parameter field changes from an anonymous struct to that named type.
Code that built the old anonymous struct literal stops compiling, but that field never produced a value.

Parameters that collapse to a single type (asc | desc | null) are unchanged, and make generate changes no existing example.
New examples/union/anyof-query-params has the reproduction spec from the issue and a test that the value reaches GetQuery() for both the nested and the flattened union.

Fixes #177