Add Array.CreateFilled<T> overloads

[SkyDevLab]

Summary

Implements the approved API enhancement from #121477:

namespace System;

public abstract partial class Array
{
    public static T[] CreateFilled<T>(int length, T value);
    public static T[] CreateFilled<T>(int length, Func<int, T> factory);
}

Design & Implementation Details

• CreateFilled<T>(int length, T value):
  • Validates length >= 0 via ArgumentOutOfRangeException.ThrowIfNegative(length).
  • Returns cached empty array [] (Array.Empty<T>()) for length == 0.
  • Uses GC.AllocateUninitializedArray<T>(length) to bypass zeroing overhead for unmanaged/value types >= 2048 bytes while safely zero-initializing reference types.
  • Fills the array using new Span<T>(array).Fill(value), taking advantage of hardware/SIMD vectorization.
• CreateFilled<T>(int length, Func<int, T> factory):
  • Validates length >= 0 via ArgumentOutOfRangeException.ThrowIfNegative(length).
  • Validates factory != null via ArgumentNullException.ThrowIfNull(factory).
  • Returns cached empty array [] for length == 0.
  • Allocates via GC.AllocateUninitializedArray<T>(length) and sequentially populates each element array[i] = factory(i).
  • Guarantees memory and GC safety even if factory throws midway during iteration.
• Updated reference assembly contract in src/libraries/System.Runtime/ref/System.Runtime.cs.
• Added comprehensive unit tests covering empty arrays, negative lengths, null factory validation, value types (small and >2048 bytes), reference types, structs with references, sequential indexing, reference instance distinctness, and exception propagation in src/libraries/System.Runtime/tests/System.Runtime.Tests/System/ArrayTests.cs.

Fixes #121477

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[teo-tsirpanis]

Duplicate of #134745.

[SkyDevLab]

Duplicate of #134745.

Thanks for pointing that out. I’ll take a look at #134745 and see if any of the additional test coverage or implementation details from my PR can be useful there.

[SkyDevLab]

Closing this PR in favor of #134745 since it was opened first. Looking forward to seeing the new Array.CreateFilled API landed in .NET!