perf: use stable sha256 cache keys for summarizer requests

JobTrackerApi.Tests/OwnershipGuardTests.cs

@@ -1,5 +1,6 @@
 using System.Reflection;
 using JobTrackerApi.Controllers;
+using JobTrackerApi.Services;
 using Xunit;
 
 namespace JobTrackerApi.Tests;
@@ -19,4 +20,19 @@ public sealed class OwnershipGuardTests
         var method = typeof(CorrespondenceController).GetMethod("FindOwnedMessageAsync", BindingFlags.NonPublic | BindingFlags.Instance);
         Assert.NotNull(method);
     }
+
+    [Fact]
+    public void Summarizer_service_uses_sha256_cache_key_builder()
+    {
+        var method = typeof(SummarizerService).GetMethod("BuildCacheKey", BindingFlags.NonPublic | BindingFlags.Static);
+        Assert.NotNull(method);
+
+        var first = (string)method!.Invoke(null, new object[] { "example text", 150, 30 })!;
+        var second = (string)method.Invoke(null, new object[] { "example text", 150, 30 })!;
+        var different = (string)method.Invoke(null, new object[] { "different text", 150, 30 })!;
+
+        Assert.Equal(first, second);
+        Assert.StartsWith("summ:", first);
+        Assert.NotEqual(first, different);
+    }
 }