fix: KrxDataService GET method + correct endpoint (pykrx-openapi compatible)

- Changed HTTP method: POST → GET
- Changed base URL: https://openapi.krx.co.kr → http://data-dbg.krx.co.kr
- Changed endpoint: /svc/sample/apis/idx/krx_dd_trd → /svc/apis/sto/stk_bydd_trd
- Query params: basDd in URL (not JSON body)
- Response parsing: OutBlock_1 field (pykrx-openapi format)
- Stub fallback: Still active when KRX_OPENAPI env var empty

Addresses: WBS optimization Step 4 (API reliability).
Code is compatible with pykrx-openapi implementation.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/KArtSell.Modules.ModelOperations/ShadowRun/Services/KrxDataService.cs

@@ -19,8 +19,8 @@ public sealed class KrxDataService : IKrxDataService
     private const int MaxRetries = 3;
     private const int InitialBackoffMs = 100;
     private const int MaxBackoffMs = 30000;
-    private const string KrxApiBaseUrl = "https://openapi.krx.co.kr";  // From appsettings: ExternalApis:KrxOpenApi:BaseUrl
-    private const string KrxApiEndpoint = "/svc/sample/apis/idx/krx_dd_trd";
+    private const string KrxApiBaseUrl = "http://data-dbg.krx.co.kr";  // Stock price API (from pykrx-openapi)
+    private const string KrxApiEndpoint = "/svc/apis/sto/stk_bydd_trd";  // KOSPI daily trading endpoint
 
     private static readonly Action<ILogger, string, DateOnly, DateOnly, Exception?> LogFetchingOhlcv =
         LoggerMessage.Define<string, DateOnly, DateOnly>(
@@ -180,20 +180,13 @@ public sealed class KrxDataService : IKrxDataService
         // Fetch each trading day in range
         for (var date = startDate; date <= endDate; date = date.AddDays(1))
         {
-            // KRX API (spec): POST /svc/apis/idx/krx_dd_trd with JSON body {"basDd":"YYYYMMDD"}
-            var endpoint = $"{KrxApiBaseUrl}{KrxApiEndpoint}";
+            // KRX API (spec): GET /svc/apis/sto/stk_bydd_trd with query param basDd=YYYYMMDD
+            var endpoint = $"{KrxApiBaseUrl}{KrxApiEndpoint}?basDd={date:yyyyMMdd}";
 
             try
             {
-                var requestBody = new { basDd = date.ToString("yyyyMMdd") };
-                var jsonContent = new StringContent(
-                    System.Text.Json.JsonSerializer.Serialize(requestBody),
-                    System.Text.Encoding.UTF8,
-                    "application/json");
-
-                var request = new HttpRequestMessage(HttpMethod.Post, endpoint);
+                var request = new HttpRequestMessage(HttpMethod.Get, endpoint);
                 request.Headers.Add("AUTH_KEY", apiKey);
-                request.Content = jsonContent;
 
                 var response = await _httpClient.SendAsync(request, cancellationToken);
 
@@ -245,9 +238,15 @@ public sealed class KrxDataService : IKrxDataService
     {
         try
         {
-            var response = JsonSerializer.Deserialize<KrxPriceResponse>(krxResponse);
-            var items = response?.Response?.Body?.Items ?? new List<PriceItem>();
-            return JsonSerializer.Serialize(items);
+            using var doc = JsonDocument.Parse(krxResponse);
+            var root = doc.RootElement;
+
+            if (root.TryGetProperty("OutBlock_1", out var outBlock))
+            {
+                return outBlock.GetRawText();
+            }
+
+            return "[]";
         }
         catch
         {