fix(kiro): handle empty content in Claude format assistant messages

Problem:
- PR #181 fixed empty content for OpenAI format (kiro_openai_request.go)
- But Claude format (kiro_claude_request.go) was not fixed
- OpenCode uses Claude format (/v1/messages endpoint)
- When assistant messages have only tool_use (no text), content becomes empty
- This causes 'Improperly formed request' errors from Kiro API

Example of problematic message format:
{
  "role": "assistant",
  "content": [
    {"type": "tool_use", "id": "...", "name": "todowrite", "input": {...}}
  ]
}

Solution:
- Add empty content fallback in BuildAssistantMessageStruct (Claude format)
- Same fix as PR #181 applied to kiro_openai_request.go

Fixes compaction failures for OpenCode + Quotio + CLIProxyAPIPlus + Kiro stack

internal/translator/kiro/claude/kiro_claude_request.go

@@ -883,8 +883,21 @@ func BuildAssistantMessageStruct(msg gjson.Result) KiroAssistantResponseMessage
 		contentBuilder.WriteString(content.String())
 	}
 
+	// CRITICAL FIX: Kiro API requires non-empty content for assistant messages
+	// This can happen with compaction requests where assistant messages have only tool_use
+	// (no text content). Without this fix, Kiro API returns "Improperly formed request" error.
+	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: assistant content was empty, using default: %s", finalContent)
+	}
+
 	return KiroAssistantResponseMessage{
-		Content:  contentBuilder.String(),
+		Content:  finalContent,
 		ToolUses: toolUses,
 	}
 }