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/common/message_merge.go

@@ -10,6 +10,7 @@ import (
 // MergeAdjacentMessages merges adjacent messages with the same role.
 // This reduces API call complexity and improves compatibility.
 // Based on AIClient-2-API implementation.
+// NOTE: Tool messages are NOT merged because each has a unique tool_call_id that must be preserved.
 func MergeAdjacentMessages(messages []gjson.Result) []gjson.Result {
 	if len(messages) <= 1 {
 		return messages
@@ -26,6 +27,12 @@ func MergeAdjacentMessages(messages []gjson.Result) []gjson.Result {
 		currentRole := msg.Get("role").String()
 		lastRole := lastMsg.Get("role").String()
 
+		// Don't merge tool messages - each has a unique tool_call_id
+		if currentRole == "tool" || lastRole == "tool" {
+			merged = append(merged, msg)
+			continue
+		}
+
 		if currentRole == lastRole {
 			// Merge content from current message into last message
 			mergedContent := mergeMessageContent(lastMsg, msg)