fix: resolve false "plan limit reached" error for cloud models

[Dang-Ver]

Summary

• Fix incorrect model_platform mapping for cloud models routed through LiteLLM proxy — non-GPT models (Gemini, Claude, Minimax) were using native provider SDKs (gemini, anthropic) instead of openai-compatible-model, causing protocol mismatches that were misinterpreted as budget errors
• Remove overly broad " 429" heuristic in error_format.py that falsely classified HTTP 429 rate limits as insufficient_quota errors, and add a separate rate_limit_exceeded handler
• Add missing cloud models (GPT-5/5.1/5.2/5-mini, Gemini 3 Flash, Claude Sonnet 4-5) to backend ModelType enum

Test plan

• Select Minimax M2.5 as cloud model and send a message — should no longer show "plan limit reached"
• Select Gemini 3 Pro as cloud model and send a message — should work without false quota error
• Select GPT 5.2 as cloud model and send a message — should work as before
• Select GPT 4.1 as cloud model and send a message — verify no regression
• Trigger an actual rate limit (429) and verify the new "Rate limit exceeded" message appears instead of "exceeded your quota"

[Dang-Ver]

@Wendong-Fan.
plz help me with this issue