fix(worker): guard timestamp-format derivation against pathological culture patterns

src/ZB.MOM.WW.MxGateway.Worker.Tests/MxAccess/MxAccessEventMapperTests.cs

@@ -303,6 +303,55 @@ public sealed class MxAccessEventMapperTests
         Assert.Equal(DateTime.SpecifyKind(localWall, DateTimeKind.Local).ToUniversalTime(), utc);
     }
 
+    /// <summary>
+    ///     Verifies format derivation survives a culture whose long-time pattern
+    ///     hides an "ss" inside quoted literal text — the case where blindly
+    ///     inserting ".fff" at the first "ss" produces a format that mangles the
+    ///     value or is outright malformed, and a malformed format makes exact
+    ///     parsing throw rather than report failure. Derivation must reject such
+    ///     a candidate at build time, so parsing here stays exception-free and
+    ///     the culture's own published shape still reads as local wall-clock
+    ///     time. Uses a culture no other test touches, because the format cache
+    ///     compares cultures by name.
+    /// </summary>
+    [Fact]
+    public void TryParseSourceTimestamp_WithLiteralSecondsInCulturePattern_ParsesWithoutThrowing()
+    {
+        CultureInfo pathological = new("en-ZA")
+        {
+            DateTimeFormat =
+            {
+                ShortDatePattern = "M/d/yyyy",
+                LongTimePattern = "'sss' HH:mm:ss",
+            },
+        };
+
+        CultureInfo original = CultureInfo.CurrentCulture;
+        try
+        {
+            CultureInfo.CurrentCulture = pathological;
+            DateTime localWall = new(2026, 3, 26, 13, 38, 22, DateTimeKind.Unspecified);
+
+            // The culture's published shape: "3/26/2026 sss 13:38:22".
+            string published = localWall.ToString(
+                pathological.DateTimeFormat.ShortDatePattern + " " + pathological.DateTimeFormat.LongTimePattern,
+                pathological);
+
+            Assert.True(MxAccessEventMapper.TryParseSourceTimestamp(published, out DateTime utc));
+            Assert.Equal(DateTimeKind.Utc, utc.Kind);
+            Assert.Equal(DateTime.SpecifyKind(localWall, DateTimeKind.Local).ToUniversalTime(), utc);
+
+            // Shapes this culture cannot describe must still fail quietly rather
+            // than throw out of the derived formats.
+            Assert.False(MxAccessEventMapper.TryParseSourceTimestamp("not a timestamp", out _));
+            Assert.False(MxAccessEventMapper.TryParseSourceTimestamp("sss sss sss", out _));
+        }
+        finally
+        {
+            CultureInfo.CurrentCulture = original;
+        }
+    }
+
     /// <summary>
     ///     The parse chain exactly as it stood before PERF-20 added the
     ///     exact-format stage, used as the parity oracle above.