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/rewind.go

@@ -224,6 +224,7 @@ func (service *Service) applyRunRewindToConversation(conversation *ConversationF
 	conversation.Entries = nil
 	conversation.NextEntrySeq = 1
 	conversation.NextTurnSeq = 1
+	conversation.ImportedTurnIDs = rewindImportedTurnPrefix(conversation.ImportedTurnIDs, decision)
 	appendEntriesInPlace(conversation, appendReplacementRunEntries(decision.PrefixEntries, entries))
 	applyRunRewindConversationState(conversation, intent, turnSeq)
 	deriveConversationLoopState(conversation)
@@ -269,10 +270,30 @@ func applyRunRewindMetadata(conversation *ConversationFile, source *Conversation
 		if source.TokenDetailsMaxTokens > 0 {
 			conversation.TokenDetailsMaxTokens = source.TokenDetailsMaxTokens
 		}
+		decision := runRewindDecision{TargetTurnSeq: turnSeq}
+		if intent.ConversationState != nil {
+			decision.HasClientTurnCount = true
+			decision.ClientTurnCount = len(intent.ConversationState.GetTurns())
+		}
+		conversation.ImportedTurnIDs = rewindImportedTurnPrefix(source.ImportedTurnIDs, decision)
 	}
 	applyRunRewindConversationState(conversation, intent, turnSeq)
 }
 
+func rewindImportedTurnPrefix(importedTurnIDs [][]byte, decision runRewindDecision) [][]byte {
+	keep := decision.TargetTurnSeq - 1
+	if decision.HasClientTurnCount {
+		keep = int64(decision.ClientTurnCount)
+	}
+	if keep <= 0 || len(importedTurnIDs) == 0 {
+		return nil
+	}
+	if keep > int64(len(importedTurnIDs)) {
+		keep = int64(len(importedTurnIDs))
+	}
+	return cloneByteSlices(importedTurnIDs[:keep])
+}
+
 func (service *Service) logRunRewindDecision(requestID string, conversationID string, eventName string, decision runRewindDecision) {
 	if service == nil || !decision.Evaluated {
 		return