Perplexity's model fields are inconsistent per model: Claude Sonnet 5 reports the real slug in user_selected_model with display_model "turbo", while GLM-5.2 reports the inverse. Pick whichever field is present and not "turbo", and fall back to the requested model when both are missing or "turbo".
Also add glm_5_2 to KNOWN_MODELS.
Refs #7 (follow-up comment)