Vulnerability Explain - Upgrade default model to Claude Sonnet 4.5

TL;DR

The Vulnerability Explain feature currently still has the default model of 3.7. Since we 3.7 will be deprecated, we want to change this default model to 4.5

Acceptance Criteria

• explain_vulnerability now has a default model of claude_sonnet_4_5_20250929

Implementation Details

This change needs to be made in the AI Gateway Repository, at https://gitlab.com/gitlab-org/modelops/applied-ml/code-suggestions/ai-assist/-/blob/431ec280d36cc282394c99a561b11af93cc993f6/ai_gateway/model_selection/unit_primitives.yml:125

Testing Approach

• Unit tests
• Integration tests
• Manual validation steps

Estimate