Count tokens for Anthropic Messages API format.

This endpoint follows the Anthropic Messages API token counting specification. It accepts the same parameters as the /v1/messages endpoint but returns token counts instead of generating a response.

Example usage:

curl -X POST "http://localhost:4000/v1/messages/count_tokens?beta=true"       -H "Content-Type: application/json"       -H "Authorization: Bearer your-key"       -d '{
    "model": "claude-3-sonnet-20240229",
    "messages": [{"role": "user", "content": "Hello Claude!"}]
  }'

Returns: {"input_tokens": <number>}