Replace CollectionsMarshal.AsSpan with IList<T> indexing in ToFastArray

[LeaFrock]

Motivation

Closes #660.

Changes

• Replace CollectionsMarshal.AsSpan with IList<T> indexing in ToFastArray(ICollection<TSource>? source)
• Move SpanExtensions.ToFastArray(ReadOnlySpan<TSource> source) to GenericsExtensions
• Remove GenericsExtensions.ToFastArray(T[]? source) when the ROS overload is enough
• Move GenericsExtensions from Helpers to Extensions folder & namespace

Checklist

• Code compiles without warnings (TreatWarningsAsErrors is enabled)
• Tests pass locally (dotnet test)
• New code has test coverage
• No breaking changes to public API (or documented in PR description)

[imperugo]

Thanks @LeaFrock. The change itself is right and I've verified the edge case I was most worried about: a null array still returns an empty result on both paths — through ReadOnlySpan<T> (null array converts to an empty span) and through ICollection<T> (the is not { Count: > 0 } pattern covers null). Behaviour is unchanged, and since both GenericsExtensions and SpanExtensions are internal, the namespace move breaks nothing for consumers.

I pushed a merge commit to this branch — here's why.

The PR was showing 2712 changed lines in CacheClientTestBase.cs for what is actually a two-line using change. The file was being converted from CRLF to LF wholesale, which buried the real change and would have wiped git blame for a 1357-line test file.

That wasn't your doing. The repo had no .gitattributes and mixed line endings — 16 files were CRLF, the other ~100 LF — so whoever touches one of those files from a different platform flips the whole thing. It was going to keep happening to every contributor.

So I fixed the cause on master first (9d6f6d6): added a .gitattributes normalizing to LF in the repository and native in the working tree, with .bat/.cmd/.ps1/.sln pinned to CRLF and .sh to LF, then ran git add --renormalize .. Purely mechanical — git diff -w --ignore-cr-at-eol is empty across all 18 touched files.

Then I merged that master into this branch. The only conflict was the using block in CacheClientTestBase.cs, which I resolved in favour of your version (Core.Helpers → Core.Extensions, and dropping the redundant using StackExchange.Redis; — the test namespace nests under it, so it resolves without).

Result: the diff went from +1408/-1444 to +54/-90, and CacheClientTestBase.cs is now 4 lines instead of 2712. Nothing else changed. Please git pull before your next commit on this branch.

Verified locally after the merge: clean build on all TFMs with TreatWarningsAsErrors, and 1754 + 26 tests green on net10.0 (two consecutive full runs).

One observation while I was in there, unrelated to this PR: AddAsyncDateTimeOffset_WithTags_CorrectTaggedKey_Async passes in the full suite but fails when run in isolation with --filter, so it depends on state left by earlier tests. That's pre-existing on master, not something you introduced — I'll open a separate issue.

Ready to merge from my side. It ships with v13.5.

[imperugo]

Merging. Thanks @LeaFrock — this ships in v13.5.