feat: add StepFun Step 3.5 Flash on OpenRouter

[cantalupo555]

Summary

• Add StepFun Step 3.5 Flash model and its free variant to OpenRouter provider
• Values sourced from the OpenRouter /api/v1/models API and model page
• Supersedes feat: Support StepFun (step-3.5-flash) via OpenRouter #778 with corrected values based on review feedback
• Pricing: $0.10/$0.30 input/output, cache read $0.02 (free variant: $0.00)
• Context: 256,000 tokens, max output: 256,000 tokens
• Modality: text→text only

Test plan

• bun validate passes successfully
• Models appear in generated output with correct IDs stepfun/step-3.5-flash and stepfun/step-3.5-flash:free
• No duplicate PR for these models