fix: address Codex review for structured output (#36) (#38)

- Include error feedback user turn in mergedMessages to maintain
  alternating user/assistant roles required by Anthropic API
- Use explicit undefined check instead of ?? for structured merge
  to preserve null as a valid structured output value

src/agent/agent.ts

@@ -335,10 +335,7 @@ export class Agent {
       ? firstAttemptError.message
       : String(firstAttemptError)
 
-    const retryMessages: LLMMessage[] = [
+    const errorFeedbackMessage: LLMMessage = {
-      ...originalMessages,
-      ...result.messages,
-      {
       role: 'user' as const,
       content: [{
         type: 'text' as const,
@@ -350,14 +347,21 @@ export class Agent {
           'Please try again. Respond with ONLY valid JSON, no other text.',
         ].join('\n'),
       }],
-      },
+    }
+
+    const retryMessages: LLMMessage[] = [
+      ...originalMessages,
+      ...result.messages,
+      errorFeedbackMessage,
     ]
 
     const retryResult = await runner.run(retryMessages, runOptions)
     this.state.tokenUsage = addUsage(this.state.tokenUsage, retryResult.tokenUsage)
 
     const mergedTokenUsage = addUsage(result.tokenUsage, retryResult.tokenUsage)
-    const mergedMessages = [...result.messages, ...retryResult.messages]
+    // Include the error feedback turn to maintain alternating user/assistant roles,
+    // which is required by Anthropic's API for subsequent prompt() calls.
+    const mergedMessages = [...result.messages, errorFeedbackMessage, ...retryResult.messages]
     const mergedToolCalls = [...result.toolCalls, ...retryResult.toolCalls]
 
     try {

src/orchestrator/orchestrator.ts

@@ -845,7 +845,7 @@ export class OpenMultiAgent {
           messages: [...existing.messages, ...result.messages],
           tokenUsage: addUsage(existing.tokenUsage, result.tokenUsage),
           toolCalls: [...existing.toolCalls, ...result.toolCalls],
-          structured: result.structured ?? existing.structured,
+          structured: result.structured !== undefined ? result.structured : existing.structured,
         })
       }