Add Synthetic provider

[blackbird-merula]

Adds basic support for Synthetic. Works for me (tested with GLM 4.6).

[alvinunreal]

Thanks @blackbird-merula -
Providers got confusing I think and something to fix but synthetic should work with openrouter provider and base_url.
Can you check if that's the case? Seem it's openai chat completion compatible endpoint, which is openrouter in this case in tmuxai, and open is new responses api.