Enhance checkpoint handling and error management in forwarder

- Added flushing of assistant text during provider completion to ensure no output is lost on transport failure.
- Updated checkpoint blob synchronization tests to validate behavior under various conditions, including terminal and non-terminal states.
- Introduced new functions for managing checkpoint terminal actions, improving clarity and maintainability of the code.
- Implemented additional tests for imported blob handling and conversation state restoration, ensuring robustness in data integrity across operations.

internal/backend/forwarder/interrupted_output_test.go

@@ -2,6 +2,7 @@ package forwarder
 
 import (
 	"encoding/json"
+	"errors"
 	"strings"
 	"testing"
 )
@@ -125,6 +126,53 @@ func TestCancelPersistsInterruptedProviderOutputIdempotently(t *testing.T) {
 	}
 }
 
+func TestGenericProviderFailurePersistsAccumulatedOutput(t *testing.T) {
+	service, stream, _ := testCheckpointBlobProjection(t)
+	conversation, _, _, err := service.snapshotCheckpointConversation(stream)
+	if err != nil {
+		t.Fatalf("snapshotCheckpointConversation() error = %v", err)
+	}
+	if _, err := service.store.SaveConversationWithEntries(stream.ConversationID, conversation, conversation.Entries); err != nil {
+		t.Fatalf("SaveConversationWithEntries() error = %v", err)
+	}
+
+	stream.mu.Lock()
+	stream.CurrentModelCallID = "model-call-1"
+	stream.ProviderActive = true
+	stream.ProviderAccumulatedText = "partial answer before transport failure"
+	stream.Status = StreamStatusStreaming
+	stream.Phase = TurnPhaseProviderRunning
+	stream.mu.Unlock()
+	if err := service.handleProviderDoneEvent(stream, &streamProviderEvent{
+		Done: true,
+		Err:  errors.New("transport failed"),
+	}); err != nil {
+		t.Fatalf("handleProviderDoneEvent() error = %v", err)
+	}
+
+	persisted, err := service.store.LoadConversation(stream.ConversationID)
+	if err != nil {
+		t.Fatalf("LoadConversation() error = %v", err)
+	}
+	foundPartialOutput := false
+	for _, entry := range persisted.Entries {
+		if entry.Kind != "assistant_text" {
+			continue
+		}
+		var payload assistantTextPayload
+		if err := json.Unmarshal(entry.Payload, &payload); err != nil {
+			t.Fatalf("decode assistant entry: %v", err)
+		}
+		if payload.Text == "partial answer before transport failure" {
+			foundPartialOutput = true
+			break
+		}
+	}
+	if !foundPartialOutput {
+		t.Fatal("generic provider failure discarded accumulated assistant output")
+	}
+}
+
 func TestCancelPreservesPersistedTurnActivityWithoutLiveAccumulator(t *testing.T) {
 	service, stream, _ := testCheckpointBlobProjection(t)
 	conversation, _, _, err := service.snapshotCheckpointConversation(stream)