.NET: Bug: Anthropic connector doesn't work for VertexAI

[Vifill]

Describe the bug
The Anthropic VertexAI needs the anthropic-version to be passed in the body (as anthropic_version) instead of the header, as is done with the Anthropic API.

(See documentation here: https://docs.anthropic.com/en/api/claude-on-vertex-ai)
In Vertex, anthropic_version is passed in the request body (rather than as a header), and must be set to the value vertex-2023-10-16.

This is not done in the connector logic, it reuses the AnthropicRequest model which doesn't have the anthropic_version in the body.

When trying to use the AddAnthropicVertextAIChatCompletion out of the box it returns

{
  "type" : "error",
  "error" : {
    "type" : "invalid_request_error",
    "message" : "anthropic_version: Field required"
  }
}

Sadly this seems to be a hard blocker since I see no good way to add this into the body, but I would love if anyone had a workaround.

UPDATE: After trying a hacky workaround by intercepting the request and editing the json before sending it I noticed a new error. The 'model' property is not allowed, so that also needs to be removed from the body.

Additional context
Not sure if it's a bug or intented, but unlike the GeminiVertexAi connector, the AddAnthropicVertextAIChatCompletion (it also has a typo) isn't creating the full endpoint for you. So you need to create the full URL like so:

var endpoint =  new Uri($"https://{location}-aiplatform.googleapis.com/v1/projects/{projectId}/locations/{location}/publishers/anthropic/models/{_modelId}:rawPredict");
builder.AddAnthropicVertextAIChatCompletion(_modelId, () => new ValueTask<string>(bearerKey), endpoint);

Which I think could be improved to be more intuitive, and be similar to the Gemini VertexAI connector.

[Vifill]

After trying a hacky workaround by intercepting the request and editing the json before sending it I noticed a new error. The 'model' property is not allowed, so that also needs to be removed from the body.

https://docs.anthropic.com/en/api/claude-on-vertex-ai
In Vertex, model is not passed in the request body. Instead, it is specified in the Google Cloud endpoint URL.