fix(e2e): send phase 8 follow-up as interrupt

[chocobar]

Bug

Helix Drone build 5019 timed out in Zed E2E protocol lifecycle Phase 8. The phase claims to send an interrupt but called SendChatMessage, whose test helper sends interrupt=false. Under one timing, the normal follow-up became a separate Zed-originated turn with a generated request ID, so the strict Phase 8 completion check never observed its expected interrupt request ID.

Fix

Send the Phase 8 follow-up through SendChatMessageWithInterrupt(..., true) with the phase's request ID. Keep the strict completion assertion. Other phases continue to use their existing send path.

Tests

• E2E test-server go test ./... with a temporary module file pointing at the local Helix checkout
• git diff --check

Full Zed E2E validation requires CI. This PR must be merged before the Helix ZED_COMMIT pin is published.