When all accounts fail with HTTP 500/503 errors (e.g., Google API returning
'Unknown Error' for Claude models on large conversations), the proxy now
attempts to use a fallback model if --fallback is enabled.
This enables graceful degradation when:
- All accounts are exhausted due to 5xx errors (not just rate limits)
- Claude models fail on very large conversations
- The API has temporary issues with specific models
The fallback uses the existing MODEL_FALLBACK_MAP configuration:
- claude-opus-4-5-thinking -> gemini-3-pro-high
- claude-sonnet-4-5-thinking -> gemini-3-flash
Relates to #88
0b477c2552