Fix for tool_calls finish reason when last streamed message is empty

[dkalinowski]

🛠 Summary

CVS-182445

[Copilot]

Pull request overview

Fixes OpenAI-compatible streaming serialization so finish_reason is correctly reported as "tool_calls" even when the final streamed chunk is empty (a known behavior with some streamers where the last callback may produce "" but still carry STOP).

Changes:

• Add per-stream state tracking (toolCallsDetectedInStream) to remember whether tool calls were emitted earlier in the stream.
• Update serializeStreamingChunk() to map STOP to "tool_calls" if tool calls were detected in any prior chunk.
• Add a regression test covering the “tool call ended with NONE, followed by empty STOP chunk” scenario.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/test/http_openai_handler_test.cpp	Adds a regression test ensuring "finish_reason":"tool_calls" is emitted when an empty STOP chunk follows tool-call streaming.
src/llm/apis/openai_completions.hpp	Introduces toolCallsDetectedInStream state to track tool-call presence across streamed chunks.
src/llm/apis/openai_completions.cpp	Sets the state when tool-call deltas appear and uses it when mapping the final streamed finish_reason.