Restore struct? default to false to suppress x-struct population

[luisgabrielroldan]

What

Restore the ActiveProspect fork patch that defaults :struct? to false in build_schema/2 (lib/open_api_spex.ex).

Why

The upstream sync (#4, through v3.22.3) reverted this fork-specific default back to upstream's true. As a result, the x-struct vendor extension is now auto-populated with fully-qualified Elixir module names (e.g. Elixir.ClaimerWeb.Schemas.BadRequest) in generated OpenAPI specs, where the fork previously left it null.

This was surfaced while migrating trustedform-api's dependency reference from tf-changes-rebased to master (prep for Elixir 1.19 / OTP 28): regenerating make api-docs showed every schema's x-struct flip from null to a module name.

Effect

• x-struct returns to null in generated specs (no internal module names leaked into published docs).
• As before, no defstruct/@type t is generated for schema modules (gated on x-struct == __MODULE__). Verified trustedform-api does not construct or pattern-match these schema structs, so this is safe.

Diff

-        if Keyword.get(opts, :struct?, true) do
+        if Keyword.get(opts, :struct?, false) do

[luisgabrielroldan]

Closing in favor of applying struct?: false per-schema in trustedform-api, to keep this fork aligned with upstream rather than re-introducing the divergence the upstream sync removed.