Node: Guard synthetic model list compatibility - #2404

Summary

Adds a focused Node compatibility guard for HydraFusion model discovery without changing SDK implementation or generated contracts.

Coordinated changes:

• Runtime authorization and discovery: github/copilot-agent-runtime#17144
• App rollout and presentation: github/github-app#13624

The test proves the existing typed models.list surface preserves:

• arbitrary synthetic model IDs (hydrafusion and hydrafusion-max);
• the existing "powerful" picker category; and
• the response rows without filtering or coercion.

Scope

No SDK source, generated API, dependency manifest, or lockfile changes are required. Existing child-process environment, experiment assignment, and experimental-mode fields already cover the coordinated runtime and App rollout.

Validation

• Node client test suite: 166 passed
• TypeScript type-check
• ESLint
• Prettier
• Node SDK CI passed on Linux, macOS, and Windows in both default and in-process modes
• CodeQL and SDK consistency review passed

The locked @github/copilot@1.0.81-6 package is not currently published, so local validation used the available 1.0.81-1 package as a no-save install override. The repository manifest and lockfile remain unchanged.