fix(kiro): handle empty content in messages to prevent Bad Request errors

Problem:
- OpenCode's /compaction command and auto-compaction (at 80%+ context)
  sends requests that can result in empty assistant message content
- Kiro API strictly requires non-empty content for all messages
- This causes 'Bad Request: Improperly formed request' errors
- After compaction failure, the malformed message stays in history,
  breaking all subsequent requests in the session

Solution:
- Add fallback content for empty assistant messages in
  buildAssistantMessageFromOpenAI()
- Add history truncation (max 50 messages) to prevent oversized requests
- This ensures all messages have valid content before sending to Kiro API

Fixes issues with:
- /compaction command returning Bad Request
- Auto-compaction breaking sessions
- Conversations becoming unresponsive after compaction failure

internal/translator/kiro/openai/kiro_openai_request.go

@@ -576,9 +576,23 @@ func processOpenAIMessages(messages gjson.Result, modelID, origin string) ([]Kir
 		}
 	}
 
+	// Truncate history if too long to prevent Kiro API errors
+	history = truncateHistoryIfNeeded(history)
+
 	return history, currentUserMsg, currentToolResults
 }
 
+const kiroMaxHistoryMessages = 50
+
+func truncateHistoryIfNeeded(history []KiroHistoryMessage) []KiroHistoryMessage {
+	if len(history) <= kiroMaxHistoryMessages {
+		return history
+	}
+
+	log.Debugf("kiro-openai: truncating history from %d to %d messages", len(history), kiroMaxHistoryMessages)
+	return history[len(history)-kiroMaxHistoryMessages:]
+}
+
 // buildUserMessageFromOpenAI builds a user message from OpenAI format and extracts tool results
 func buildUserMessageFromOpenAI(msg gjson.Result, modelID, origin string) (KiroUserInputMessage, []KiroToolResult) {
 	content := msg.Get("content")
@@ -677,8 +691,20 @@ func buildAssistantMessageFromOpenAI(msg gjson.Result) KiroAssistantResponseMess
 		}
 	}
 
+	// CRITICAL FIX: Kiro API requires non-empty content for assistant messages
+	// This can happen with compaction requests or error recovery scenarios
+	finalContent := contentBuilder.String()
+	if strings.TrimSpace(finalContent) == "" {
+		if len(toolUses) > 0 {
+			finalContent = "I'll help you with that."
+		} else {
+			finalContent = "I understand."
+		}
+		log.Debugf("kiro-openai: assistant content was empty, using default: %s", finalContent)
+	}
+
 	return KiroAssistantResponseMessage{
-		Content:  contentBuilder.String(),
+		Content:  finalContent,
 		ToolUses: toolUses,
 	}
 }