feat(flow-chat): surface terminal failures in chat

[wsp1911]

Summary

• Persist terminal turn error messages and structured provider diagnostics through session history.
• Render localized inline failure notices in FlowChat, including expandable technical details and raw-error copy.
• Keep zero-round failed turns in the transcript and remove their notification-center error toast.

Fixes #

Type and Areas

Type:

Feature / UI/UX

Areas:

Rust core, desktop/Tauri, web UI

Motivation / Impact

Terminal failures, including failures before a model round starts, now remain visible and searchable in the conversation. Users can inspect provider, HTTP, request ID, and raw diagnostics without leaving FlowChat.

The notification center no longer shows turn-failure errors. MCP interaction and ACP permission error notifications remain unchanged.

Verification

• pnpm --dir src/web-ui run test:run src/flow_chat/components/modern/TurnFailureNoticeItem.test.tsx (2 passed)
• pnpm --dir src/web-ui run test:run src/flow_chat/services/flow-chat-manager/EventHandlerModule.test.ts (22 passed)
• pnpm run type-check:web
• pnpm run i18n:audit (passed, 0 warnings)
• git diff --check
• cargo check --workspace did not complete within the local 120-second timeout.
• Manual FlowChat interaction verification was intentionally not run.

Reviewer Notes

Session fields are optional and backward-compatible. Failed zero-round turns are retained instead of being restored to the pending queue and deleted.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, including the Rust-check timeout.
• User-facing strings and locales are updated for en-US, zh-CN, and zh-TW.