fix: remove partial agent messages on error in ACP conversation

Previously, when Send() failed, only empty agent messages were removed.
If the agent had streamed partial content before the error, that partial
message would incorrectly remain in the conversation.

Now we remove the agent message on error regardless of whether it has
content, ensuring the conversation state stays consistent.

Author: johnstcn

x/acpio/acp_conversation.go

@@ -199,9 +199,8 @@ func (c *ACPConversation) executePrompt(messageParts []st.MessagePart) {
 
 	if err != nil {
 		c.logger.Error("ACPConversation message failed", "error", err)
-		// Remove the empty streaming message on error
-		if len(c.messages) > 0 && c.messages[len(c.messages)-1].Role == st.ConversationRoleAgent &&
-			c.messages[len(c.messages)-1].Message == "" {
+		// Remove the agent's streaming message on error (may be empty or partial)
+		if len(c.messages) > 0 && c.messages[len(c.messages)-1].Role == st.ConversationRoleAgent {
 			c.messages = c.messages[:len(c.messages)-1]
 		}
 		messages := slices.Clone(c.messages)

x/acpio/acp_conversation_test.go

@@ -423,3 +423,52 @@ func Test_Messages_AreCopied(t *testing.T) {
 	// Unblock the write to let the test complete cleanly
 	close(done)
 }
+
+func Test_ErrorRemovesPartialMessage(t *testing.T) {
+	mClock := quartz.NewMock(t)
+	mock := newMockAgentIO()
+	// Block the write so we can simulate partial content before error
+	started, done := mock.BlockWrite()
+
+	conv := acpio.NewACPConversation(mock, nil, nil, nil, mClock)
+	conv.Start(context.Background())
+
+	// Send a message
+	err := conv.Send(screentracker.MessagePartText{Content: "test"})
+	require.NoError(t, err)
+
+	// Wait for write to start
+	<-started
+
+	// Should have user message + placeholder agent message
+	messages := conv.Messages()
+	require.Len(t, messages, 2)
+	assert.Equal(t, screentracker.ConversationRoleUser, messages[0].Role)
+	assert.Equal(t, screentracker.ConversationRoleAgent, messages[1].Role)
+
+	// Simulate the agent streaming partial content before the error
+	mock.SimulateChunks("partial ", "response ", "content")
+
+	// Verify partial content is in the agent message
+	messages = conv.Messages()
+	require.Len(t, messages, 2)
+	assert.Equal(t, "partial response content", messages[1].Message)
+
+	// Now configure the mock to return an error and unblock
+	mock.mu.Lock()
+	mock.writeErr = assert.AnError
+	mock.mu.Unlock()
+	close(done)
+
+	// Wait for the conversation to stabilize after the error
+	require.Eventually(t, func() bool {
+		return conv.Status() == screentracker.ConversationStatusStable
+	}, 100*time.Millisecond, 5*time.Millisecond, "status should return to stable")
+
+	// The partial agent message should be removed on error.
+	// Only the user message should remain.
+	messages = conv.Messages()
+	require.Len(t, messages, 1, "partial agent message should be removed on error")
+	assert.Equal(t, screentracker.ConversationRoleUser, messages[0].Role)
+	assert.Equal(t, "test", messages[0].Message)
+}