Adds ProviderConfig.extra_query, threaded into AsyncOpenAI(default_query)
so that Azure-style gateways requiring query params like api-version can
be configured without URL hacks.
Also updates provider_signature to track extra_query changes so per-turn
refresh rebuilds the provider when the value changes.
Addresses the extra_query portion of #4204. The max_completion_tokens
model-awareness enhancement is intentionally left separate.
