feat(minimax): Thinking-Parsing, 1h-Cache-TTL, Null-Safety, Provider-Integration

- Thinking-Streaming: Parse 'reasoning_details' (MiniMax mit reasoning_split),
  'reasoning' (Nvidia), 'reasoning_content' (DeepSeek/OpenAI-Compat),
  und inline <think>...</think>-Tags aus delta.content (Fallback)
- MiniMax: max_tokens=32768, stream_options={include_usage}, reasoning_split=true
- 1h-Prompt-Cache-TTL für API (extended-cache-ttl-2025-04-11 Header,
  Cache-Breakpoints auf System-Prompt + letztes Tool) und CLI (ENABLE_PROMPT_CACHING_1H=1)
- Null-Safety: choices/delta/tool_calls/usage null-Checks in Streaming + Non-Streaming
- finish_reason='length' → warning-Event + stopReason='max_tokens'
- MiniMax-Provider: BaseUrl, MinimaxApiKey-Setting, /api/models/minimax-Endpoint,
  Model-Dropdown, statische Fallback-Modelle, Cache-Laden mit Fallback
- Safety-Net: InvalidOperationException-Catch um SSE-Zeilen-Block

Services/ServerSettingsService.cs

@@ -37,6 +37,7 @@ public class ServerSettingsService
         new SettingDefinition( "GitHubEventScanner", "bool",   "false", "GitHub Event Scanner (Echtzeit-Monitoring öffentlicher Pushes)", "toggle"   ),
         new SettingDefinition( "HuggingFaceToken",   "string", "",      "HuggingFace Token (optional, erhöht Rate-Limits)",            "password" ),
         new SettingDefinition( "NvidiaApiKey",       "string", "",      "Nvidia API-Key (direkt statt g4f.space Proxy)",                "password" ),
+        new SettingDefinition( "MinimaxApiKey",      "string", "",      "MiniMax API-Key (für minimax-Provider im Chat)",               "password" ),
         new SettingDefinition( "ScannerKeyTypes",    "string_array", "anthropic_oauth,krea_ai", "Key-Scanner: Aktive Key-Typen",                               "checkboxes" ),
     };