Anthropic - set max_tokens based on available context

[krissetto]

Calculate max_tokens value for Anthropic requests based on the available context using the official token counting API.

The PR fixes errors such as:

400 Bad Request

{
  "type": "error",
  "error": {
    "type": "invalid_request_error",
    "message": "input length and `max_tokens` exceed context limit: 178959 + 64000 > 200000, decrease input length or `max_tokens` and try again"
  },
  "request_id":"req_011CUB2XUHWkFnQzxcKBbjpF"
}

Closes #569

[krissetto]

Right now I implemented this calculation using the official Anthropic count tokens API.

Another option would be to do some approximated calculation with some external library, and use that estimate instead of an additional API call.