Add e2e phases for the production prompt-queue path

[lukemarsden]

Summary

The WebSocket-sync e2e drove every phase through the low-level SendChatMessage
primitive. Since Helix now sends all agent messages through the session-scoped
prompt queue (see the companion helix PR), that primitive is no longer the
production send path. This adds two phases that exercise the real production
queue path (EnqueueQueuedPrompt → processPendingPromptsForSession →
processPromptQueue / processInterruptPrompt → sendQueuedPromptToSession)
against a real Zed binary.

Changes

• Phase 16 — queue busy-defer: start a turn, then enqueue an interrupt=false
  message while it streams; assert it is HELD (no concurrent interaction) and
  delivered as the next turn once idle (the concurrent-mid-turn incident, fixed).
• Phase 17 — queue interrupt: enqueue an interrupt=true message while a turn
  streams; assert it cancels the running turn (interaction → interrupted) and its
  own message is delivered.
• Phases poll the store (queue prompts get generated request_ids that don't flow
  through the completion switch) and wait for the first turn to stream before the
  second enqueue.
• go.mod/go.sum: re-synced with the current helix dependency graph.

Runs green locally (run_docker_e2e.sh, zed-agent): all 17 phases + store
validations PASSED.

Depends on the helix PR (test helper EnqueueQueuedPrompt + in-memory prompt
queue in memorystore); sandbox-versions.txt ZED_COMMIT is bumped there to
this commit.

Release Notes:

• N/A

---

🔗 Open in Helix

📋 Spec:

• Requirements
• Design
• Tasks

🚀 Built with Helix

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