fix: Tag-Feld für Sub-Agent-Messages – verhindert API 400 bei tool_use ohne tool_result

- MessageRecord: neues Tag-Feld (null=normal, preparer/review/finalizer=Sub-Agent)
- DatabaseService: Migration tag-Spalte, INSERT-Statements erweitert
- AnthropicService: RunAgentChat setzt Tag=agentType bei allen 4 SaveMessage-Aufrufen
- ChatService: BuildHistoryFromDb filtert getaggte Messages aus API-History

Sub-Agent-Messages bleiben in der DB (Frontend-Anzeige), werden aber
nie an die Claude API geschickt – löst das Problem für Preparer,
Review und Finalizer gleichermaßen.

Services/AnthropicService.cs

@@ -691,7 +691,8 @@ public class AnthropicService
                         Iteration = baseIterations + iterations,
                         Role = "assistant",
                         Type = "text",
-                        Content = $"[{tag}: {agentName}] {text}"
+                        Content = $"[{tag}: {agentName}] {text}",
+                        Tag = agentType
                     });
                 }
                 else if (blockType == "tool_use")
@@ -718,7 +719,8 @@ public class AnthropicService
                         Type = "tool_use",
                         Content = toolInput.GetRawText(),
                         ToolName = toolName,
-                        ToolCallId = toolId
+                        ToolCallId = toolId,
+                        Tag = agentType
                     });
 
                     string toolResult;
@@ -748,7 +750,8 @@ public class AnthropicService
                         Content = toolResult,
                         ToolName = toolName,
                         ToolCallId = toolId,
-                        IsError = isError
+                        IsError = isError,
+                        Tag = agentType
                     });
                 }
                 else if (blockType == "web_search_tool_result")
@@ -779,7 +782,8 @@ public class AnthropicService
                             Iteration = baseIterations + iterations,
                             Role = "assistant",
                             Type = "web_search_result",
-                            Content = JsonSerializer.Serialize(sources)
+                            Content = JsonSerializer.Serialize(sources),
+                            Tag = agentType
                         });
                     }
                 }