Support Claude on Google Vertex AI

### Problem Statement

OCR cannot use Claude over Vertex AI, because the API is a bit different[0].

[0] https://platform.claude.com/docs/en/build-with-claude/claude-on-vertex-ai

### Proposed Solution

Add a OCR_USE_ANTHROPIC_VERTEX=true to use the proper API,

or it can be autodetected using something like:
```go
isVertex := strings.Contains(cfg.URL, "aiplatform.googleapis.com")
```
(not sure about this one)

### Alternatives Considered

_No response_

### Affected Area

CLI / Commands, Review Agent / LLM interaction

### Additional Context

I can work on it, if this feature is desirable.