Agent loop responses bypass responseInterceptor; deduplicate step-message wrapping
## Summary
`getAllMessages()` bypasses `responseInterceptor` for all agent loop iterations after the first response. The root cause is duplicated logic. The step-message wrapping exists in both the interceptor (line 111, "Contacting agents..") and inline in `getAllMessages()` (line 309, "Calling agents.."). The `getAllMessages()` path uses a direct `fetch()` that never invokes the interceptor, so contributed modules cannot reliably extend response handling for agent-based assistants.
## Proposed fix
1. Extract the duplicated step-message wrapping into a shared `buildStepHtml()` helper
2. Simplify the interceptor to delegate to the helper
3. Call `responseInterceptor` in `getAllMessages()` before processing
Net result: removes duplication, unifies the loading text, and makes the interceptor work consistently.
## Environment
- Module version: 1.3.5
- Drupal version: 11.3.2
issue