ci: update claude-opus model version to 4-6 in eval configuration

Author: omkargaikwad23

evals/claude_code_model.yaml

@@ -14,7 +14,7 @@
 
 claude_code_version: "@anthropic-ai/claude-code@2.1.85"
 generator: claude_code
-model: "claude-opus-4-1@20250805"
+model: "claude-opus-4-6"
 
 use_vertex: true
 vertex_project_id: "${GOOGLE_CLOUD_PROJECT}"