Instructor fails to work with OpenRouter when using Anthropic models

[syberkitten]

What Model are you using?

• anthropic/cloaude-3-sonnet
• gpt-4o

Describe the bug
Instructor fails to work work with openrouter when using Anthropic.

base_url = 'https://openrouter.ai/api/v1'
client = instructor.from_openai(AsyncOpenAI(base_url=base_url, api_key={openrouter_api_key}))
client.chat.completions.create(**params, model='anthropic/claude-3-sonnet', extra_headers=headers, response_model=response_model)

The we get is:
` ) -> BaseModel:
message = completion.choices[0].message
assert (

      len(message.tool_calls or []) == 1

    ), "Instructor does not support multiple tool calls, use List[Model] instead."

E AssertionError: Instructor does not support multiple tool calls, use List[Model] instead.`

If

Same code as above for 'openai/gpt-4o' works perfectly for OPENAI (using openrouter) and we get a response model as expected:
client.chat.completions.create(**params, model='openai/gpt-4o', extra_headers=headers, response_model=response_model)

If you will try to use the AsyncAnthropic instead of AsyncOpenAI the error gets even weirder and there is a long stack trace, this is the outmost error, something with not reaching the proper endpoint or something.

      raise self._make_status_error_from_response(err.response) from None

E anthropic.NotFoundError

Expected behavior
Allow to use Anthropic models over openrouter wether if it by using the openai client or anthropic (which would be better since the APIs are not identical)