Provider-Abstraktion: Agents unterstützen jetzt OpenAI + Anthropic

Backend:
- ChatEndpoint: Globaler OpenAI-Fallback entfernt, Key-Check pro Provider
- TelegramService/CronjobService: Provider-Weiche statt hardcoded Anthropic
- AnthropicService: Preparer/Review/Finalizer sind Provider-aware
- OpenAiService: Neue ChatNonStreaming()-Methode für agentic loop
- ToolService: send_to_session lädt Key des Ziel-Agents
- SubAgentContext: Provider-Feld hinzugefügt
- ChatService: BuildSubAgentContext setzt Provider

Frontend:
- Key-Switch-Modal: 'Kein Key'-Option zum Entfernen der Zuweisung
- ApiKeysEndpoint: keyId='none' löscht Key-Zuweisung

Services/TelegramService.cs

@@ -15,6 +15,7 @@ public class TelegramService
     private readonly DatabaseService _db;
     private readonly ChatService _chatService;
     private readonly AnthropicService _anthropic;
+    private readonly OpenAiService _openAi;
     private readonly ServerSettingsService _settings;
 
     public TelegramService(
@@ -22,12 +23,14 @@ public class TelegramService
         DatabaseService db,
         ChatService chatService,
         AnthropicService anthropic,
+        OpenAiService openAi,
         ServerSettingsService settings)
     {
         _httpFactory = httpFactory;
         _db = db;
         _chatService = chatService;
         _anthropic = anthropic;
+        _openAi = openAi;
         _settings = settings;
     }
 
@@ -197,11 +200,23 @@ public class TelegramService
                 Content = userText
             });
 
-            var reply = await _anthropic.ChatNonStreaming(ctx, history, onIteration: async (iteration) =>
+            string reply;
+            if (!string.IsNullOrEmpty(ctx.Provider))
             {
-                if (iteration % 3 == 0)
-                    await SendChatAction(chatId, botToken, "typing");
-            });
+                reply = await _openAi.ChatNonStreaming(ctx, history, onIteration: async (iteration) =>
+                {
+                    if (iteration % 3 == 0)
+                        await SendChatAction(chatId, botToken, "typing");
+                });
+            }
+            else
+            {
+                reply = await _anthropic.ChatNonStreaming(ctx, history, onIteration: async (iteration) =>
+                {
+                    if (iteration % 3 == 0)
+                        await SendChatAction(chatId, botToken, "typing");
+                });
+            }
 
             await SendLongMessage(chatId, botToken, reply);
         }