feat: Gemini thought_signature korrekt lesen und zurücksenden

- thought_signature aus delta.tool_calls[n].extra_content.google.thought_signature lesen (korrekter Pfad)
- ThoughtSignature-Feld in MessageRecord + DB-Migration (ALTER TABLE messages ADD COLUMN thought_signature)
- SaveMessage speichert ThoughtSignature pro Tool-Call
- OpenAiService: toolCallAccumulators Tuple um ThoughtSignature erweitert
- Tool-Calls mit thought_signature werden mit extra_content.google.thought_signature zurückgesendet (nicht function.thought_signature)
- ConvertMessage (History-Rebuild) setzt thought_signature ebenfalls korrekt
- ChatService.BuildApiMessages übergibt thought_signature aus DB-Record
- Gemini-Modelle dynamisch laden statt hardcodiert
- Tool-Call-Akkumulator nutzt id als primären Key (Gemini sendet parallele Calls ohne index)
- content=null → content='' für Gemini-Kompatibilität

Services/DatabaseService.cs

@@ -135,6 +135,7 @@ public class DatabaseService
         TryAddColumn(conn, "agents", "tool_choice", "TEXT DEFAULT 'any'");
         TryAddColumn(conn, "agents", "custom_commands", "TEXT");
         TryAddColumn(conn, "messages", "is_marked", "INTEGER NOT NULL DEFAULT 0");
+        TryAddColumn(conn, "messages", "thought_signature", "TEXT");
         TryAddColumn(conn, "agents", "auto_commit", "INTEGER NOT NULL DEFAULT 0");
 
         // Migration: auto_commit von INTEGER (0/1) zu TEXT ("", "mention", "forbid")
@@ -768,8 +769,8 @@ public class DatabaseService
         using var conn = CreateConnection();
         record.CreatedAt = DateTime.UtcNow.ToString("o");
         return await conn.ExecuteScalarAsync<long>(@"
-            INSERT INTO messages (chain_id, role, type, content, tool_name, tool_call_id, is_error, iteration, attachment_count, tag, created_at)
-            VALUES (@ChainId, @Role, @Type, @Content, @ToolName, @ToolCallId, @IsError, @Iteration, @AttachmentCount, @Tag, @CreatedAt);
+            INSERT INTO messages (chain_id, role, type, content, tool_name, tool_call_id, is_error, iteration, attachment_count, tag, thought_signature, created_at)
+            VALUES (@ChainId, @Role, @Type, @Content, @ToolName, @ToolCallId, @IsError, @Iteration, @AttachmentCount, @Tag, @ThoughtSignature, @CreatedAt);
             SELECT last_insert_rowid();
         ", record);
     }