Use explicit full-path for loading MsQuic.dll on Windows (#109963)

* Use explicit full-path for loading MsQuic.dll on Windows

* Suppress IL3000 in MsQuicApi constructor

A call to `Assembly.Location` was added in a recent fix. It has `IL30000` suppressed via `#pragma warning disable`, but that only applies to the compilation of the library itself. Consumers will hit it when doing something like publishing their app as NativeAOT.

This change adds an `[UnconditionalSuppressMessage]` to the `MsQuicApi` static constructor such that `IL30000` should also be suppressed for apps consuming the runtime.

This was caught in an aspnetcore deps flow PR coming from runtime.

---------

Co-authored-by: Elinor Fung <elfung@microsoft.com>

src/libraries/System.Net.Quic/src/System/Net/Quic/Internal/MsQuicApi.cs

@@ -70,6 +70,8 @@ private MsQuicApi(QUIC_API_TABLE* apiTable)
     internal static bool Tls13ClientMayBeDisabled { get; }
 
 #pragma warning disable CA1810 // Initialize all static fields in 'MsQuicApi' when those fields are declared and remove the explicit static constructor
+    [UnconditionalSuppressMessage("SingleFile", "IL3000: Avoid accessing Assembly file path when publishing as a single file",
+        Justification = "The code handles the Assembly.Location being null/empty by falling back to AppContext.BaseDirectory")]
     static MsQuicApi()
     {
         bool loaded = false;
@@ -89,8 +91,23 @@ static MsQuicApi()
 
         if (OperatingSystem.IsWindows())
         {
-            // Windows ships msquic in the assembly directory.
-            loaded = NativeLibrary.TryLoad(Interop.Libraries.MsQuic, typeof(MsQuicApi).Assembly, DllImportSearchPath.AssemblyDirectory, out msQuicHandle);
+#pragma warning disable IL3000 // Avoid accessing Assembly file path when publishing as a single file
+            // Windows ships msquic in the assembly directory next to System.Net.Quic, so load that.
+            // For single-file deployments, the assembly location is an empty string so we fall back
+            // to AppContext.BaseDirectory which is the directory containing the single-file executable.
+            string path = typeof(MsQuicApi).Assembly.Location is string assemblyLocation && !string.IsNullOrEmpty(assemblyLocation)
+                ? System.IO.Path.GetDirectoryName(assemblyLocation)!
+                : AppContext.BaseDirectory;
+#pragma warning restore IL3000
+
+            path = System.IO.Path.Combine(path, Interop.Libraries.MsQuic);
+
+            if (NetEventSource.Log.IsEnabled())
+            {
+                NetEventSource.Info(null, $"Attempting to load MsQuic from {path}");
+            }
+
+            loaded = NativeLibrary.TryLoad(path, typeof(MsQuicApi).Assembly, DllImportSearchPath.LegacyBehavior, out msQuicHandle);
         }
         else
         {