Change default model for new sessions to sonnet-4.6

[redhat-ship-help]

Summary

Change the default model for new agentic sessions from the current default to sonnet-4.6.

Details

The default model for new sessions needs to be updated across multiple configuration layers:

• CRD default: Update the default value in components/manifests/base/crds/agenticsessions-crd.yaml (currently claude-3-7-sonnet-latest) under spec.llmSettings.model
• Backend fallback: Update any hardcoded model defaults in the backend code (components/backend/)
• UI default: Update the default selection shown in the session creation UI
• ambient-models ConfigMap: Ensure sonnet-4.6 is marked as the default model in the dynamic model configuration

Acceptance Criteria

• New sessions created without explicitly specifying a model should use sonnet-4.6
• This applies to sessions created via the UI, API, and acpctl CLI
• Existing sessions should not be affected

[github-actions]

Session session-c0a451d6-c572-4501-90d8-2e8f8e5902a0 created (phase: Running). PR will have the ambient-code:managed label.