Add [OverloadResolutionPriority(1)] to MemoryMarshal.AsBytes(Span) and Cast(Span)

[Copilot]

Description

C# 14 overload resolution changes cause MemoryMarshal.AsBytes<T>(int[]) and MemoryMarshal.Cast<TFrom, TTo>(int[]) to resolve to the ReadOnlySpan overloads instead of the Span overloads, breaking existing code that assigns the result to Span<byte>. This is the same issue that was already fixed for AsRef<T> in #122217 and #122330.

int[] bits = [];
// C# 13: compiles (resolves to Span overload)
// C# 14: error CS0029 — resolves to ReadOnlySpan overload
Span<byte> asBytes = MemoryMarshal.AsBytes<int>(bits);
Span<ulong> cast = MemoryMarshal.Cast<int, ulong>(bits);

Changes

• MemoryMarshal.cs: Added [OverloadResolutionPriority(1)] to AsBytes<T>(Span<T>) and Cast<TFrom, TTo>(Span<TFrom>), matching the existing AsRef<T>(Span<byte>) pattern
• System.Runtime.cs (ref assembly): Added corresponding attribute declarations
• Tests: Added Span_AsBytes_ImplicitSpanConversion_ReturnsMutableSpan and CastSpan_ImplicitSpanConversion_ReturnsMutableSpan, following the existing AsRef_ImplicitSpanConversion_ReturnsMutableRef pattern

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[Copilot]

Pull request overview

This PR updates System.Runtime.InteropServices.MemoryMarshal to account for C# 14 overload resolution changes by explicitly prioritizing the mutable Span<T> overloads over the ReadOnlySpan<T> overloads, preventing source-breaking overload selection in common call sites (e.g., passing an array and assigning to Span<...>).

Changes:

• Add [OverloadResolutionPriority(1)] to MemoryMarshal.AsBytes<T>(Span<T>) and MemoryMarshal.Cast<TFrom, TTo>(Span<TFrom>) in CoreLib.
• Mirror the same priority attribute additions in the System.Runtime reference assembly.
• Add new unit tests that validate arrays bind to the Span<T> overloads (so assignments to Span<...> continue to compile under C# 14).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Runtime/ref/System.Runtime.cs	Adds overload priority attributes to the public ref signatures for AsBytes(Span) and Cast(Span).
src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/MemoryMarshal.cs	Prioritizes the mutable Span<T> overloads for AsBytes and Cast to preserve pre-C#14 call-site behavior.
src/libraries/System.Memory/tests/MemoryMarshal/AsBytesSpan.cs	Adds a regression test ensuring array arguments select the Span<T> AsBytes overload.
src/libraries/System.Memory/tests/MemoryMarshal/CastSpan.cs	Adds a regression test ensuring array arguments select the Span<T> Cast overload.