[wozhenbang2004]

Related to #132.

When a Chat Completions client sends a tool-call history to an OpenAI Responses endpoint, OpenAIReqToOpenAI2 drops the assistant's tool_calls. It also sends each tool result as a message with role: "tool", losing its tool_call_id instead of producing a Responses function_call_output item.

Preserve function calls and their results as separate input items, including parallel calls and result ordering. Keep accompanying assistant string content, but omit the empty message for tool-only assistant turns. String and text-part-array tool results are supported. Existing handling of other message content and streaming response conversion is unchanged.

Verification (Go 1.27.3, Linux; fixed upstream source at 425ffca2751483baf7df13dcc15fda7b2500ff8c):

• Three new tool-history cases fail against the original production converter and pass with this change; ordinary-message and malformed-input controls pass on both.
• go test ./internal/transformer/... -count=1 passes with the complete transformer package and its logger dependency.
• go vet ./internal/transformer/... and formatting checks pass.
• A local smoke test through cx/chat.NewOpenAI2Transformer().TransformRequest preserves call IDs, multipart output, model and streaming flag.

This is converter-level verification, not a full proxy/desktop build or a live upstream-provider test. No provider API calls were made.