The module-level auxiliary_is_nous was set to True by _try_nous() and
never reset. In long-running gateway processes, once Nous was resolved
as auxiliary provider, the flag stayed True forever — even if
subsequent resolutions chose a different provider (e.g. OpenRouter).
This caused Nous product tags to be sent to non-Nous providers.
Reset the flag at the start of _resolve_auto() so only the winning
provider's flag persists.
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