Fix Random.GetItems observably breaking change in produced sequence (#…

…108018)

Developers, often in tests, rely on seeded Random instances producing the same sequence of values on every use. We made a change in .NET 9, though, that changed the sequence GetItems produces, due to employing a different algorithm. This fixes that special-case to only be used when the developer couldn't rely on the results being deterministic, namely when using either `new Random()` or `Random.Shared`. If a seed is provided or if a custom derived implementation is used, it falls back to the old behavior.

Co-authored-by: Stephen Toub <stoub@microsoft.com>

src/libraries/System.Private.CoreLib/src/System/Random.cs

@@ -199,11 +199,16 @@ public void GetItems<T>(ReadOnlySpan<T> choices, Span<T> destination)
 
             // The most expensive part of this operation is the call to get random data. We can
             // do so potentially many fewer times if:
+            // - the instance was constructed as `new Random()` or is `Random.Shared`, such that it's not seeded nor is it
+            //   a custom derived type. We don't want to observably change the deterministically-produced sequence from previous releases.
             // - the number of choices is <= 256. This let's us get a single byte per choice.
             // - the number of choices is a power of two. This let's us use a byte and simply mask off
             //   unnecessary bits cheaply rather than needing to use rejection sampling.
             // In such a case, we can grab a bunch of random bytes in one call.
-            if (BitOperations.IsPow2(choices.Length) && choices.Length <= 256)
+            ImplBase impl = _impl;
+            if ((impl is null || impl.GetType() == typeof(XoshiroImpl)) &&
+                BitOperations.IsPow2(choices.Length) &&
+                choices.Length <= 256)
             {
                 Span<byte> randomBytes = stackalloc byte[512]; // arbitrary size, a balance between stack consumed and number of random calls required
                 while (!destination.IsEmpty)