fix: re-route AgentConnectionStore through AgentConnectionCache (redux)

[lukemarsden]

Summary

Re-applies the AgentConnectionStore portion of ba7e97aea6 that was reverted in 350de991de. The revert commit explicitly flagged this as a known-unfixed race that "can be addressed separately if it ever surfaces" — it surfaced.

Problem

Two claude-agent-acp wrappers spawn for the same resumed spec-task session, each with their own claude --resume and their own chrome-devtools-mcp. One of the chrome-devtools-mcp instances loses the npx race and never registers, so the model talking to that wrapper sees Error: No such tool available: mcp__chrome-devtools__*.

Reproduced on ubuntu-external-01kq54vkjmb...:

• pstree: two sh -c "claude-agent-acp" parents, only one with chrome-devtools-mcp under it
• Zed.log.old at 16:02:47: two [ACP_SPAWN] About to spawn ACP wrapper agent_id=AgentId("claude-acp") lines, but only one [ACP_DEDUP] No cached connection line — confirming the second spawn skipped the cache

The race: agent panel restoration on a resumed session calls AgentConnectionStore::request_connection for claude-acp at the same instant external_websocket_sync::thread_service::open_existing_thread_sync does. Both paths independently call server.connect(). The earlier fix only routed thread_service through the cache, leaving the cross-path race open.

Fix

Route AgentConnectionStore::start_connection through AgentConnectionCache::request_connection so UI-driven connects share a single AgentConnection with external_websocket_sync::thread_service for the same (project, agent_id).

Why the original revert reason no longer applies

The revert (350de991de) said E2E_AGENTS="zed-agent,claude" ./run_docker_e2e.sh round 2 (claude) phase 1 timed out with 0 events. With current main (post the stale-flag and other thread_service fixes from #44 and #45 on this fork), that E2E now passes both rounds with this change re-applied.

##################################################
  FINAL RESULTS
##################################################
  [zed-agent] PASSED
  [claude] PASSED
  [store] PASSED

Test plan

• E2E_AGENTS="zed-agent,claude" ./run_docker_e2e.sh passes (verified locally)
• In a fresh spec task: pstree shows only one claude-agent-acp parent per session
• Zed.log shows [ACP_DEDUP] Reusing connection for the second caller
• mcp__chrome-devtools__* tools work consistently

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