refactor: Backend-Deduplizierung – Parameter-Objekte, Shared Helpers, KeyCheckDispatcher

- ToolDefinitionOptions + ToolExecutionContext ersetzen lange Parameterlisten in ToolService
- AppendToLastMessage-Helper in ChatService eliminiert 5x duplizierte Logik
- OpenAiService: ResolveProviderConfig + BuildApiMessages + ConvertToOpenAiTools statt doppeltem Code
- Tool-Definitionen aus einer Quelle (Anthropic→OpenAI Konvertierung statt hardcodierter Liste)
- KeyCheckDispatcher zentralisiert Key-Check-Dispatch (ApiKeysEndpoint, KeyScanner, GitHubEventScanner)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Services/KeyPatterns.cs

@@ -1,6 +1,8 @@
 using System;
 using System.Linq;
+using System.Net.Http;
 using System.Text.RegularExpressions;
+using System.Threading.Tasks;
 
 namespace Gott.Services;
 
@@ -102,6 +104,19 @@ public static class KeyPatterns
         return type == "anthropic_oauth" || type == "anthropic_api" || type == "anthropic_unknown";
     }
 
+    /// <summary>
+    /// Mappt einen Key-Typ auf den Provider-Namen für den Checker.
+    /// Gibt null zurück wenn kein Checker verfügbar ist.
+    /// </summary>
+    public static string GetCheckerProvider(string type)
+    {
+        if (IsAnthropic(type)) return "anthropic";
+        if (type == "openai_project" || type == "openai_service" || type == "openai_legacy") return "openai";
+        if (type == "google_api") return "google";
+        if (type == "stripe_live" || type == "stripe_test") return "stripe";
+        return null;
+    }
+
     /// <summary>Menschenlesbarer Name für den Key-Typ.</summary>
     public static string DisplayName(string type)
     {