Handle empty Gemini responses, extend firstN tests, update docs and session test comment

[swuecho]

Motivation

• Prevent unexpected nil responses from the Gemini model path and improve error messaging when the LLM returns nothing.
• Increase coverage for firstN by adding edge and Unicode cases.
• Clarify API usage for artifact retrieval and client-side execution in the docs.
• Avoid brittle timestamp equality checks in the chat session test by asserting updated fields instead.

Description

• Change Gemini regular response handling in model_gemini_service.go to return an explicit error when llmAnswer is nil and to set AnswerId only after confirming a non-nil response, and fix a typo in the error message to response.
• Add additional unit test cases to Test_firstN in util_test.go to cover Unicode characters, empty strings, zero length, and emoji inputs.
• Update chat_session_service_test.go to stop asserting strict UpdatedAt equality and instead verify the updated fields (topic), with a clarifying comment.
• Update documentation in docs/artifact_gallery_en.md and docs/artifact_gallery_zh.md to use the messages endpoint (/api/uuid/chat_messages/chat_sessions/${sessionId}) and to state that code execution is handled client-side rather than via a backend /api/artifacts/:id/execute endpoint.

Testing

• Ran unit tests with go test ./... which included Test_firstN and the chat session tests, and they all passed.
• Verified that the updated firstN test cases execute and validate Unicode, empty, zero-length, and emoji behaviors successfully.

---

Codex Task