Use safe writes in Xoshiro NextBytes

[EgorBo]

Note

This PR description was generated by GitHub Copilot.

Summary

• Rewrite xoshiro Random.NextBytes bulk writes to use BitConverter.TryWriteBytes
• Replace the unsafe partial-tail byte-copy path with a collection-literal scratch span and BitConverter.TryWriteBytes
• Remove the now-unneeded unsafe method declarations and MemoryMarshal using directives

Validation

• build.cmd clr.corelib+clr.nativecorelib+libs.pretest -rc checked
• dotnet.cmd build /t:test src\libraries\System.Runtime\tests\System.Runtime.Extensions.Tests\System.Runtime.Extensions.Tests.csproj -p:XUnitClassName=System.Tests.RandomTests -p:TargetFramework=net11.0-windows -v:minimal (121 tests, 0 failed)

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-runtime
See info in area-owners.md if you want to be subscribed.

[Copilot]

Pull request overview

This PR refactors the Xoshiro-based Random.NextBytes implementations in System.Private.CoreLib to avoid unsafe code by switching bulk and tail writes to BitConverter.TryWriteBytes.

Changes:

• Replace MemoryMarshal.Write bulk writes with BitConverter.TryWriteBytes and add Debug.Assert for the expected success path.
• Replace unsafe tail byte-copy logic with a scratch span plus TryWriteBytes, removing the need for unsafe.
• Remove the System.Runtime.InteropServices using directives made unnecessary by the refactor.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/libraries/System.Private.CoreLib/src/System/Random.Xoshiro256StarStarImpl.cs	Refactors NextBytes(Span<byte>) to use TryWriteBytes for bulk and tail writes, removing unsafe pointer usage.
src/libraries/System.Private.CoreLib/src/System/Random.Xoshiro128StarStarImpl.cs	Same refactor pattern for the 32-bit Xoshiro implementation’s NextBytes(Span<byte>).

[EgorBo]

@MihuBot