fix: keep model selector visible when loading fails

[Jaspersol]

摘要

当 /api/models 请求失败时，模型选择器现在会保留在界面中，并显示具体错误信息。对于开发服务器冷启动、服务重启或 cwd allow-list 尚未同步等瞬时故障，前端会按退避策略自动重试，避免模型选择器静默隐藏。

变更内容

• 为模型列表请求增加重试退避间隔：2 秒、5 秒、10 秒。
• 解析非成功响应中的 JSON error 字段，并将具体错误展示给用户。
• 对非 JSON 错误响应回退到包含 HTTP 状态码的错误信息。
• 请求被取消时立即停止重试，避免组件卸载后继续更新状态。
• 模型加载成功后保持原有模型列表和模型错误状态更新逻辑。

影响范围

• 文件：hooks/useAgentSession.ts
• 用户可见行为：模型列表加载失败时，模型选择器不再被静默隐藏；临时失败会自动恢复。

验证

• npm 测试 — 844 tests 通过。
• tsc --noEmit。
• npm run lint。
• git diff --check。

English

Title

Fix the model selector being hidden when model loading fails

Summary

When /api/models fails, the model selector now remains visible and displays a useful error message. For transient failures caused by a dev-server cold start, server restart, or a cwd allow-list race, the client retries with backoff instead of leaving the selector hidden prematurely.

Changes

• Added model-list retry backoff delays of 2 seconds, 5 seconds, and 10 seconds.
• Extracted the error field from non-success JSON responses and surfaced it to the UI.
• Falls back to an HTTP-status-based message for non-JSON error responses.
• Stops retries immediately when the request is aborted, preventing updates after component unmount.
• Preserved the existing model-list and model-error state updates after successful loading.

Scope

• File: hooks/useAgentSession.ts
• User-visible behavior: the model selector is no longer silently hidden when model loading fails; transient failures can recover automatically.

Verification

• npm test — 844 tests passed。
• tsc --noEmit。
• npm run lint。
• git diff --check。