fix(kiro): Handle tool results correctly in OpenAI format translation

Fix three issues in Kiro OpenAI translator that caused "Improperly formed request"
errors when processing LiteLLM-translated requests with tool_use/tool_result:

1. Skip merging tool role messages in MergeAdjacentMessages() to preserve
   individual tool_call_id fields

2. Track pendingToolResults and attach to the next user message instead of
   only the last message. Create synthetic user message when conversation
   ends with tool results.

3. Insert synthetic user message with tool results before assistant messages
   to maintain proper alternating user/assistant structure. This fixes the case
   where LiteLLM translates Anthropic user messages containing only tool_result
   blocks into tool role messages followed by assistant.

Adds unit tests covering all tool result handling scenarios.

internal/translator/kiro/openai/kiro_openai_request.go

@@ -469,6 +469,11 @@ func processOpenAIMessages(messages gjson.Result, modelID, origin string) ([]Kir
 		}
 	}
 
+	// Track pending tool results that should be attached to the next user message
+	// This is critical for LiteLLM-translated requests where tool results appear
+	// as separate "tool" role messages between assistant and user messages
+	var pendingToolResults []KiroToolResult
+
 	for i, msg := range messagesArray {
 		role := msg.Get("role").String()
 		isLastMessage := i == len(messagesArray)-1
@@ -480,6 +485,10 @@ func processOpenAIMessages(messages gjson.Result, modelID, origin string) ([]Kir
 
 		case "user":
 			userMsg, toolResults := buildUserMessageFromOpenAI(msg, modelID, origin)
+			// Merge any pending tool results from preceding "tool" role messages
+			toolResults = append(pendingToolResults, toolResults...)
+			pendingToolResults = nil // Reset pending tool results
+
 			if isLastMessage {
 				currentUserMsg = &userMsg
 				currentToolResults = toolResults
@@ -505,6 +514,24 @@ func processOpenAIMessages(messages gjson.Result, modelID, origin string) ([]Kir
 
 		case "assistant":
 			assistantMsg := buildAssistantMessageFromOpenAI(msg)
+
+			// If there are pending tool results, we need to insert a synthetic user message
+			// before this assistant message to maintain proper conversation structure
+			if len(pendingToolResults) > 0 {
+				syntheticUserMsg := KiroUserInputMessage{
+					Content: "Tool results provided.",
+					ModelID: modelID,
+					Origin:  origin,
+					UserInputMessageContext: &KiroUserInputMessageContext{
+						ToolResults: pendingToolResults,
+					},
+				}
+				history = append(history, KiroHistoryMessage{
+					UserInputMessage: &syntheticUserMsg,
+				})
+				pendingToolResults = nil
+			}
+
 			if isLastMessage {
 				history = append(history, KiroHistoryMessage{
 					AssistantResponseMessage: &assistantMsg,
@@ -524,7 +551,7 @@ func processOpenAIMessages(messages gjson.Result, modelID, origin string) ([]Kir
 		case "tool":
 			// Tool messages in OpenAI format provide results for tool_calls
 			// These are typically followed by user or assistant messages
-			// Process them and merge into the next user message's tool results
+			// Collect them as pending and attach to the next user message
 			toolCallID := msg.Get("tool_call_id").String()
 			content := msg.Get("content").String()
 
@@ -534,9 +561,21 @@ func processOpenAIMessages(messages gjson.Result, modelID, origin string) ([]Kir
 					Content:   []KiroTextContent{{Text: content}},
 					Status:    "success",
 				}
-				// Tool results should be included in the next user message
-				// For now, collect them and they'll be handled when we build the current message
-				currentToolResults = append(currentToolResults, toolResult)
+				// Collect pending tool results to attach to the next user message
+				pendingToolResults = append(pendingToolResults, toolResult)
+			}
+		}
+	}
+
+	// Handle case where tool results are at the end with no following user message
+	if len(pendingToolResults) > 0 {
+		currentToolResults = append(currentToolResults, pendingToolResults...)
+		// If there's no current user message, create a synthetic one for the tool results
+		if currentUserMsg == nil {
+			currentUserMsg = &KiroUserInputMessage{
+				Content: "Tool results provided.",
+				ModelID: modelID,
+				Origin:  origin,
 			}
 		}
 	}