[codex] TASK(WORKSPACE-A157014E8DCFDAFA)-4: fix(chat): return 503 with actionable message when no CLI worker is connected and surface it in Web UI

[zhangweijian97]

Linear issue: TASK(WORKSPACE-A157014E8DCFDAFA)-4

This PR was created by the devos.ing ADHD (Agentic Development Hub & Daemon) workflow.

Includes:

• plan + implement session output
• separate review/testing session

[zhangweijian97]

Scoped review for TASK(WORKSPACE-A157014E8DCFDAFA)-4 on PR #250 is failing due to one concrete regression.

Required fix

1. chat-routes.ts missing notFound import causes runtime ReferenceError

• Repro: hit a missing-session route such as DELETE /api/chat/sessions/<missing-id> or GET /api/chat/sessions/<missing-id>/messages.
• Observed: code reaches notFound("Chat session not found"), but notFound is not imported in packages/server/src/http/chat-routes.ts, so runtime throws instead of returning 404.
• Expected: missing-session routes return structured 404 JSON responses.
• Fix expectation: re-add notFound to the ./http-utils import list (or consistently replace those calls with another valid 404 helper).
• Verify: run targeted tests (for example bun test packages/server/tests/chat-routes.test.ts) and confirm missing-session branches return 404 without runtime exceptions.

Validation context

Coverage for the new success-goal behavior is otherwise present (503 response test and Web API error-detail surfacing test). Full bun test is not green in this environment due to existing unrelated suite/module-resolution failures.

[zhangweijian97]

devos.ing implementation feedback for TASK(WORKSPACE-A157014E8DCFDAFA)-4

Review/testing summary:
Scoped review against TASK(WORKSPACE-A157014E8DCFDAFA)-4 found one concrete regression in the changed server route code: chat-routes.ts removed the notFound import but still calls notFound(...), which will throw at runtime on missing-session paths instead of returning a 404 response. Coverage for the new success-goal behavior is otherwise present (503 server response test and Web API error-detail surfacing test), but bun test failed in this environment (many existing unrelated suite failures/module-resolution issues), so full workspace validation is not green.

Fix instructions for the implementation agent:
Address every item below, use each body as the repair checklist, and rerun the listed verification checks.

Bugs to fix:

1. chat-routes missing notFound import causes runtime ReferenceError
   Failing command/repro: On PR branch, call a chat-session path that should return not-found (for example DELETE /api/chat/sessions/<missing-id> or GET /api/chat/sessions/<missing-id>/messages) through the server app.
   Observed: Route execution reaches notFound("Chat session not found"), but notFound is not imported in packages/server/src/http/chat-routes.ts, causing a ReferenceError instead of a 404 response.
   Expected: Missing-session chat endpoints should return a structured 404 error response, not throw.
   Likely files/code path: packages/server/src/http/chat-routes.ts (import block and handleSessionRoute/handleMessagesRoute return branches).
   Fix expectation: Re-add notFound to the ./http-utils import list (or replace calls with another valid 404 helper consistently) so all missing-session branches return proper HTTP 404 bodies.
   Verification: Run targeted route tests that hit missing-session branches (for example bun test packages/server/tests/chat-routes.test.ts) and confirm those cases return 404 JSON responses without runtime exceptions.