fix(runtime): recover missing replies after SSE disconnect

[XingYu-Zhong]

Summary

• Treat local SSE terminated stream reads as transient disconnects so the main process reconnects from the last delivered sequence instead of ending the stream.
• Reconcile completed turns from persisted Kun thread detail when the renderer receives completion without any visible assistant text.
• Add regression coverage for terminated SSE reconnects and completion-time assistant reply recovery.

Changes

• Broadened runtime SSE transient error classification to cover abrupt local stream termination/socket errors.
• Added a guarded completion reconciliation path in the chat event sink that only fetches thread detail when the completed turn has no local assistant text, and skips state replacement if the user switched threads or started another turn.
• Hardened turn completion notification against non-browser test environments.

Tests

• npm test -- src/main/runtime-sse-ipc.test.ts src/renderer/src/store/chat-store-runtime.test.ts
• npm run typecheck
• npm run build
• npx eslint src/main/runtime-sse-ipc.ts src/main/runtime-sse-ipc.test.ts src/renderer/src/store/chat-store-runtime.ts src/renderer/src/store/chat-store-runtime.test.ts
• npm run lint currently fails on existing baseline react-hooks/rules-of-hooks in src/renderer/src/components/chat/message-timeline-process.tsx:248 plus existing hook dependency warnings.

Fixes #653