Stop Copilot turns aborted during session opening

Check the owner turn signal after asynchronous Copilot session opening, before sending a prompt. Cover the delayed-open abort with a stub session regression.

Assistant-model: GPT-6 Sol
Assistant-workflow: harness-stack (run 4c435721-e04e-4740-a68b-44f5281e0e40)
Assistant-duration: unmeasured converged, estimated 15m
Assistant-verification: Bun focused regression passed: delayed-open abort failed before fix with one send and passed after with no send, settled turn and cleanup
Assistant-verification: Bun unit passed: mise exec -- bun test (1568 pass, 2 PostgreSQL skips)
Assistant-verification: Bun ci passed: mise exec -- bun run ci (existing unrelated lint warning)
Assistant-verification: Bun typecheck passed: mise exec -- bun run types (workspaces and E2E)
Assistant-verification: Playwright E2E passed: flock /tmp/chopin-e2e.lock mise exec -- bun run e2e --workers=1 (230/230)
Assistant-verification: qlty metrics and smells passed: adapter.ts analyzed; existing complexity and duplication findings deferred
Co-authored-by: Alex Lavaee <lavaman131@github.com>