Agentic autofix generation for code scanning alerts [Private Preview]

### Value Prop

Users can now assign code scanning alerts—individually or within a security campaign—to an agentic version of Autofix that autonomously creates pull requests to fix the problem. Unlike the current Autofix, this agentic version uses tool calling and advanced agent behaviors to iteratively test, validate, and refine fixes before presenting them to developers.

This release covers the initial public preview, focused on campaigns and individual alerts. A separate GA release will follow.

### Expected Outcome

- Increased rate of security alert remediation across customer organizations
- Higher Autofix acceptance rates compared to current non-agentic version
- Reduced time from alert creation to merged fix
- Decreased manual effort spent on security debt reduction initiatives