fix(openai): avoid empty choices stream heartbeat

[NgoQuocViet2001]

💻 变更类型 | Change Type

• ✨ feat
• 🐛 fix
• ♻️ refactor
• 💄 style
• 👷 build
• ⚡️ perf
• 📝 docs
• 🔨 chore

🔀 变更说明 | Description of Change

Fixes #429.

OpenAI Chat Completions streaming currently sends keep-alive traffic as both an SSE comment and a JSON chat.completion.chunk with choices: []. Some strict OpenAI-compatible clients index choices[0] for every data frame, so that heartbeat chunk can raise IndexError: list index out of range and abort an otherwise healthy stream.

This change keeps the heartbeat as a valid SSE comment only:

• preserve : keep-alive\n\n and flush behavior so idle streams still stay warm
• stop emitting a JSON data frame with an empty choices array
• update the regression test to assert keep-alives do not produce JSON data frames or [DONE]

Duplicate check: searched open PRs/issues for choices / empty choices / IndexError; the currently open PRs (#422, #407, #405) cover unrelated read-cache, WebUI env, and multi-turn work.

📝 补充信息 | Additional Information

Validation run locally with portable Go 1.26.0:

• gofmt -w internal/httpapi/openai/chat/chat_stream_runtime.go internal/httpapi/openai/chat/chat_stream_runtime_test.go
• go test ./internal/httpapi/openai/chat -run TestChatStreamKeepAliveUsesCommentOnly -count=1
• go test ./internal/httpapi/openai/chat -count=1
• go test ./internal/httpapi/openai/... -count=1
• go test ./... -count=1
• git diff --check

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