chore: add changeset and regenerate openapi spec for vertex CUA

The vertex CUA change made VertexModelConfigObject.auth and .providerOptions
optional but left the generated OpenAPI spec stale, which would have carried
the old required-fields contract into the Stainless-generated SDKs.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: viviana-sudo

.changeset/vertex-cua-support.md

@@ -0,0 +1,5 @@
+---
+"@browserbasehq/stagehand": minor
+---
+
+Add Vertex AI support for Google Computer Use Agents. The `vertex` provider (explicit, or a `vertex/` model prefix) routes Google CUA models through Vertex AI, initializing `@google/genai` in Vertex mode with service-account, express-mode API key, or ambient ADC auth. `VertexModelConfigObject.auth` and `.providerOptions` are now individually optional, since express keys and ADC need neither.

packages/server-v3/openapi.v3.yaml

@@ -379,16 +379,15 @@ components:
           type: string
           const: vertex
         auth:
-          description: Vertex provider authentication configuration
+          description: Vertex provider authentication configuration. Optional when using
+            an express-mode apiKey or ambient application default credentials
           $ref: "#/components/schemas/GoogleServiceAccountAuth"
         providerOptions:
           description: Vertex provider-specific model configuration
           $ref: "#/components/schemas/VertexModelProviderOptions"
       required:
         - modelName
         - provider
-        - auth
-        - providerOptions
       additionalProperties: false
     AzureEntraModelConfigObject:
       type: object