fix: preserve dynamic model headers

[bcho]

What changed

• Preserve generated per-model headers in the dynamic catalog cache.
• Propagate assistant API failures instead of returning blank successful output.
• Verify cached headers reach the model registry request configuration.

Why

Dynamic GitHub Copilot models replaced built-ins without required headers such as Editor-Version. Copilot rejected prompts with HTTP 400, while the CLI suppressed the assistant error and printed a blank line.

Validation

• npm test
• npm run check
• Built and installed the SEA binary locally
• Refreshed the dynamic catalog
• Confirmed a real GitHub Copilot prompt returns a response