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

Endpoints/ModelsEndpoint.cs

@@ -15,6 +15,7 @@ public static class ModelsEndpoint
         app.MapGet("/api/models/pollinations", HandlePollinationsModels);
         app.MapGet("/api/models/gemini",       HandleGeminiModels);
         app.MapGet("/api/models/openai",       HandleOpenAiModels);
+        app.MapGet("/api/models/minimax",      HandleMinimaxModels);
         app.MapPost("/api/models/refresh",     HandleRefresh);
     }
 
@@ -50,6 +51,14 @@ public static class ModelsEndpoint
         return Results.Json(new { models });
     }
 
+    private static IResult HandleMinimaxModels(OpenAiService openAiService)
+    {
+        var models = openAiService.GetMinimaxModels()
+            .Select(m => new { id = m.Id, displayName = m.DisplayName })
+            .ToList();
+        return Results.Json(new { models });
+    }
+
     private static async Task<IResult> HandleRefresh(OpenAiService openAiService)
     {
         await openAiService.RefreshModelsFromApis();