[chocobar]

Change

• Replace Phase 15's provider-dependent >=40 message_added threshold with a check for assistant content visible before completion.
• Keep the existing gap and final-window checks, so delayed streaming still fails.
• Cover duplicate snapshots, the failed Helix build 4644 content pattern, and terminal-only output.
• Pass provider keys to the Docker test container without placing their values in process arguments.

Verification

• Prime: go test -mod=mod -modfile=phase15-test.mod ./... passed for the E2E test server package.
• Prime: live Claude E2E passed all 17 phases, including Phase 15, with claude-agent-acp@0.82.0 (exit 0).
• bash -n run_docker_e2e.sh and git diff --check passed.

Pinned by helixml/helix#3309.