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>

Endpoints/SessionEndpoint.cs

@@ -298,12 +298,12 @@ public static class SessionEndpoint
         var tools = new List<object>();
         if (hasTools)
         {
-            var allTools = toolService.GetToolDefinitions(
-                includeFileTools: hasWorkspace,
-                includeMemory: includeMemory,
-                includeUserChoice: agent.AllowUserChoice,
-                includeGitHubSearch: includeGitHubSearch,
-                includeSerperSearch: includeSerperSearch);
+            var allTools = toolService.GetToolDefinitions(new ToolDefinitionOptions
+            {
+                IncludeFileTools = hasWorkspace, IncludeMemory = includeMemory,
+                IncludeUserChoice = agent.AllowUserChoice,
+                IncludeGitHubSearch = includeGitHubSearch, IncludeSerperSearch = includeSerperSearch
+            });
             if (planMode)
                 allTools = toolService.FilterReadOnlyTools(allTools);
             tools.AddRange(allTools);