GET /v3/apps/:guid/routes accepts include but ignores it #5560

GET /v3/routes?include=domain returns the domains under included; GET /v3/apps/:guid/routes?include=domain returns 200 with no included at all.

Both actions build a RoutesListMessage, which allows and validates include (routes_list_message.rb:12,19):

validates_with IncludeParamValidator, valid_values: ['domain', 'space', 'space.organization', 'route_policies']

index turns it into decorators (routes_controller.rb:46-50):

decorators = []
decorators << IncludeRouteDomainDecorator if IncludeRouteDomainDecorator.match?(message.include)
decorators << IncludeSpaceDecorator if IncludeSpaceDecorator.match?(message.include)
decorators << IncludeOrganizationDecorator if IncludeOrganizationDecorator.match?(message.include)
decorators << IncludeRoutePoliciesDecorator if IncludeRoutePoliciesDecorator.match?(message.include)

index_by_app (routes_controller.rb:299-327) validates the same message and renders PaginatedListPresenter without decorators:, so a valid include is accepted and dropped.

The v3 docs for this endpoint don't list include (routes/_list_for_app.md.erb), so either fix is consistent with them: apply the same decorators in index_by_app (and document include), or reject include there. Applying them seems the expected behaviour, since every other route list supports it. Happy to send a PR for whichever you prefer.

Found with a contract check of the v3 docs, the OpenAPI spec and the request messages (see #5496).