feat(providers): add MiniMax M3 to Synthetic provider

[patrik-kuehl]

This PR adds MiniMax M3 to Synthetic's model catalog.

It also marks MiniMax M3 as an open-weighted model since its weights are now publicly available.

This is the response body from Synthetic:

{
  "provider": "synthetic",
  "always_on": true,
  "id": "hf:MiniMaxAI/MiniMax-M3",
  "hugging_face_id": "MiniMaxAI/MiniMax-M3",
  "name": "MiniMaxAI/MiniMax-M3",
  "input_modalities": [
    "text",
    "image"
  ],
  "output_modalities": [
    "text"
  ],
  "context_length": 524288,
  "max_output_length": 65536,
  "pricing": {
    "prompt": "$0.0000006",
    "completion": "$0.0000012",
    "image": "0",
    "request": "0",
    "input_cache_reads": "$0.0000006",
    "input_cache_writes": "0"
  },
  "created": 1781222400,
  "quantization": "fp8",
  "supported_sampling_parameters": [
    "temperature",
    "top_k",
    "top_p",
    "repetition_penalty",
    "frequency_penalty",
    "presence_penalty",
    "stop",
    "seed"
  ],
  "supported_features": [
    "tools",
    "json_mode",
    "structured_outputs",
    "reasoning"
  ],
  "openrouter": {
    "slug": "minimax/minimax-m3"
  },
  "datacenters": [
    {
      "country_code": "US"
    }
  ]
}

Synthetic's model catalog seems to be a bit outdated in general. I'll see if I can clean it up in the following weeks.