[Api] Add test for regression

Add test that appears to relate to GHSA-8785-wc3w-h8q6, although it passes before and after that change was reverted.

Author: martincostello

test/OpenTelemetry.Tests/Trace/Propagation/TraceContextPropagatorTests.cs

@@ -243,6 +243,14 @@ public void TryExtractTracestate_SingleHeaderRejectsDuplicateLongKeys()
         Assert.False(TraceContextPropagator.TryExtractTracestate([$"{key}=1,{key}=2"], out _));
     }
 
+    [Fact]
+    public void Extract_AllowsKeyNamesInsideEarlierValues()
+    {
+        // Regression test for GHSA-8785-wc3w-h8q6.
+        Assert.Equal("foo=bar,bar=1", CallTraceContextPropagator("foo=bar,bar=1"));
+        Assert.Equal("foo=bar,bar=1", CallTraceContextPropagator(["foo=bar", "bar=1"]));
+    }
+
     [Fact]
     public void TryExtractTracestate_NullCollectionReturnsEmpty()
     {