Rename standard conversation mode to 会话

[cinderzhan]

Summary

• Rename the standard conversation mode from “默认” / “原生会话” to “会话” in the sidebar switcher.
• Use the same name in the switcher's title and accessible label, market preview and help text, and panel failure message.
• Update focused sidebar copy assertions.

Validation

• npx vitest run test/workbench-client.test.mjs — 136 passed
• npm run typecheck — passed
• git show --format= --check codex/rename-workbench-conversation-mode — passed

This PR contains only the copy change and its focused test updates.