fix(kiro): handle empty content in current user message for compaction

Problem:
- PR #186 fixed empty content for assistant messages and history user messages
- But current user message (isLastMessage == true) was not fixed
- When user message contains only tool_result (no text), content becomes empty
- This causes 'Improperly formed request' errors from Kiro API
- Compaction requests from OpenCode commonly have this pattern

Solution:
- Move empty content check BEFORE the isLastMessage branch
- Apply fallback content to ALL user messages, not just history
- Add DefaultUserContentWithToolResults and DefaultUserContent constants

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

internal/translator/kiro/claude/kiro_claude_request.go

@@ -608,18 +608,22 @@ func processMessages(messages gjson.Result, modelID, origin string) ([]KiroHisto
 
 		if role == "user" {
 			userMsg, toolResults := BuildUserMessageStruct(msg, modelID, origin)
+			// CRITICAL: Kiro API requires content to be non-empty for ALL user messages
+			// This includes both history messages and the current message.
+			// When user message contains only tool_result (no text), content will be empty.
+			// This commonly happens in compaction requests from OpenCode.
+			if strings.TrimSpace(userMsg.Content) == "" {
+				if len(toolResults) > 0 {
+					userMsg.Content = kirocommon.DefaultUserContentWithToolResults
+				} else {
+					userMsg.Content = kirocommon.DefaultUserContent
+				}
+				log.Debugf("kiro: user content was empty, using default: %s", userMsg.Content)
+			}
 			if isLastMessage {
 				currentUserMsg = &userMsg
 				currentToolResults = toolResults
 			} else {
-				// CRITICAL: Kiro API requires content to be non-empty for history messages too
-				if strings.TrimSpace(userMsg.Content) == "" {
-					if len(toolResults) > 0 {
-						userMsg.Content = "Tool results provided."
-					} else {
-						userMsg.Content = "Continue"
-					}
-				}
 				// For history messages, embed tool results in context
 				if len(toolResults) > 0 {
 					userMsg.UserInputMessageContext = &KiroUserInputMessageContext{

internal/translator/kiro/common/constants.go

@@ -37,6 +37,14 @@ const (
 	// that have no content at all. Kiro API requires non-empty content.
 	DefaultAssistantContent = "I understand."
 
+	// DefaultUserContentWithToolResults is the fallback content for user messages
+	// that have only tool_result (no text). Kiro API requires non-empty content.
+	DefaultUserContentWithToolResults = "Tool results provided."
+
+	// DefaultUserContent is the fallback content for user messages
+	// that have no content at all. Kiro API requires non-empty content.
+	DefaultUserContent = "Continue"
+
 	// KiroAgenticSystemPrompt is injected only for -agentic models to prevent timeouts on large writes.
 	// AWS Kiro API has a 2-3 minute timeout for large file write operations.
 	KiroAgenticSystemPrompt = `