Add Synthorai as a model provider

[cuihuan]

Requesting a new model provider for Synthorai (https://synthorai.io), following the same pattern as the recently-added Kimi (#2141) and DashScope (#2073/#1866) providers.

Synthorai is an OpenAI/Anthropic-compatible LLM gateway routing to 122 models across 11 upstream providers (Claude, GPT, Gemini, GLM, Kimi, DeepSeek, Qwen, etc.) at direct upstream pricing, no markup. API docs: https://synthorai.io/docs

Since it's a plain OpenAI-compatible surface, I'd expect this to fit under src/agentscope/model/ alongside the existing OpenAI-compatible model classes with just a different default base_url (https://synthorai.io/v1). Happy to put together the PR if this is something the team would take -- flagging as an idea first per the contributing guide.

[cuihuan]

Belated but important disclosure: I'm affiliated with Synthorai — this request adds support for our own product, it isn't me reporting demand from elsewhere. I should have said so in the original post; flagging it now so the ask is judged on its merits with that context on the table.

No expectation of priority attached. Happy to do the implementation work myself, and equally happy to take "not now" for an answer.