Update vertex.mdx (#2123)

Include example using Anthropic's Claude Sonnet 4 with the Vertex AI API
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----

> [!IMPORTANT]
> Add example for Anthropic's Claude Sonnet 4 model with Vertex AI API
in `vertex.mdx`.
> 
>   - **Documentation**:
> - Adds example for using Anthropic's Claude Sonnet 4 model with Vertex
AI API in `vertex.mdx`.
> - Specifies `model` as "claude-sonnet-4" and includes
`anthropic_version` and `base_url` options.
> 
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src="https://img.shields.io/badge/Ellipsis-blue?color=175173">](https://www.ellipsis.dev?ref=BoundaryML%2Fbaml&utm_source=github&utm_medium=referral)<sup>
for 019a58d. You can
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summary. It will automatically update as commits are pushed.</sup>

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Author: villagab4

fern/03-reference/baml/clients/providers/vertex.mdx

@@ -410,3 +410,15 @@ client<llm> VertexLlama {
 }
 ```
 
+For anthropic  
+
+```baml
+client<llm> VertexClaudeSonnet {
+  provider vertex-ai
+  options {
+    model "claude-sonnet-4"
+    anthropic_version "${ANTHROPIC_VERSION}"
+    base_url "https://${LOCATION}-aiplatform.googleapis.com/v1/projects/${PROJECT_ID}/locations/${LOCATION}/publishers/anthropic/models"
+  }
+}
+```