fix: skip empty assistant message in tool call translation (#2132)

When assistant has tool_calls but no text content, the translator
emitted an empty message into the Responses API input array before
function_call items. The API then couldn't match function_call_output
to its function_call by call_id, returning:

  No tool output found for function call ...

Only emit assistant messages that have content parts. Tool-call-only
messages now produce function_call items directly.

Added 9 tests for tool calling translation covering single/parallel
calls, multi-turn conversations, name shortening, empty content
edge cases, and call_id integrity.

internal/translator/codex/openai/chat-completions/codex_openai_request.go

@@ -197,7 +197,12 @@ func ConvertOpenAIRequestToCodex(modelName string, inputRawJSON []byte, stream b
 					}
 				}
 
-				out, _ = sjson.SetRaw(out, "input.-1", msg)
+				// Don't emit empty assistant messages when only tool_calls
+				// are present — Responses API needs function_call items
+				// directly, otherwise call_id matching fails (#2132).
+				if role != "assistant" || len(gjson.Get(msg, "content").Array()) > 0 {
+					out, _ = sjson.SetRaw(out, "input.-1", msg)
+				}
 
 				// Handle tool calls for assistant messages as separate top-level objects
 				if role == "assistant" {