openapi3gen: inline embedded struct with options-only JSON tag

[mvanhorn]

Summary

openapi3gen emits a nested object property instead of inlining an embedded struct when the embedded field's JSON tag carries options but no name, e.g. *NestedStruct \json:",omitempty"`. This inlines such fields, matching encoding/json`, so the embedded struct's fields are promoted into the parent schema.

Background

encoding/json flattens an anonymous struct field unless its JSON tag gives it an explicit name; json:",omitempty" has options but no name, so it is still inlined. openapi3gen's appendFields only inlined anonymous fields when the tag was completely empty (jsonTag == ""), so a nameless-but-optioned tag fell through and became a nested object keyed by the Go field name. EnriqueL8 (#924) and a second commenter both hit this with the common ?include=a,b,c query-parameter pattern, where the wrapper struct is embedded with json:",omitempty".

The fix reads the tag's name segment with strings.Cut(jsonTag, ",") and, when the name is empty (and the tag is not -), recurses via appendFields exactly as the empty-tag path does. Explicitly-named anonymous fields still become nested objects, and json:"-" is still skipped. Added regression tests for both pointer and value embedded structs with an options-only tag.

Closes #924

[fenollp]

Thanks!

[mvanhorn]

Thanks @fenollp - embedded structs with options-only JSON tags inline correctly in openapi3gen now.