fix(kis-api): Null reference 검증 강화 (토큰 응답 처리)

KisApiClient.TryGetAccessTokenAsync()의 null 참조 경고 제거.
- 토큰 응답 본문 존재 여부 검증
- TryGetValue 기반 안전한 파싱
- access_token 필수 필드 검증

Build: 0 errors, 0 warnings ✅

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

src/dotnet/QuantEngine.Infrastructure/Services/KisApiClient.cs

@@ -174,8 +174,15 @@ public class KisApiClient : IKisApiClient
             response.EnsureSuccessStatusCode();
 
             var tokenData = await response.Content.ReadFromJsonAsync<Dictionary<string, object>>();
-            var accessToken = tokenData["access_token"]?.ToString() ?? throw new InvalidOperationException("No access_token in response");
+            if (tokenData == null) throw new InvalidOperationException("Token response body is empty");
-            var expiresInStr = tokenData.ContainsKey("expires_in") ? tokenData["expires_in"]?.ToString() : "86400";
+
+            if (!tokenData.TryGetValue("access_token", out var tokenObj) || tokenObj == null)
+                throw new InvalidOperationException("No access_token in response");
+            var accessToken = tokenObj.ToString()!;
+
+            var expiresInStr = tokenData.TryGetValue("expires_in", out var expiresObj) && expiresObj != null
+                ? expiresObj.ToString()
+                : "86400";
             var expiresInSec = int.TryParse(expiresInStr, out var seconds) ? seconds : 86400;
             var expiresAt = DateTime.UtcNow.AddSeconds(expiresInSec);