`validateResponseQuality` flagged any response with `content: null` as
empty and triggered a false-positive 502 combo fallback, even when the
upstream returned the entire answer in `reasoning_content`. This
affected every reasoning model that omits `content` by design:
moonshotai/Kimi-K2.5-TEE, zai-org/GLM-5-TEE, zai-org/GLM-4.7-TEE,
DeepSeek-R1 and Qwen-thinking variants via Chutes.ai, Nvidia, and other
OpenAI-compatible gateways.
Treat a non-empty `reasoning_content` (or its legacy `reasoning` alias)
as valid content. Empty/whitespace-only strings still fall through to
the existing empty-content rejection so we don't weaken the guard.
Backward compat verified: regular content-only and tool_calls-only
responses still validate without change.
