Fix Seq.empty rendering as "EmptyEnumerable" in serializers

[apoorvdarshan]

Summary

Fixes #17864

• Seq.empty used a custom DU type EmptyEnumerable<'T> that serializers detected as an F# union, rendering "EmptyEnumerable" instead of []
• Changed Seq.empty to delegate to System.Linq.Enumerable.Empty<'T>(), which returns a standard .NET type
• Updated Seq.concat optimization from :? EmptyEnumerable<'T> to :? ICollection<'T> when Count = 0 for broader empty collection detection
• EmptyEnumerable<'T> type is kept unchanged for backward compatibility

Test plan

• Added test verifying Seq.empty is not a discriminated union type (FSharpType.IsUnion returns false)
• Added test verifying Seq.concat with empty elements still produces correct results
• All 147 existing Seq module tests pass

[github-actions]

❗ Release notes required

@apoorvdarshan,

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[T-Gro]

Please see #19322 (comment) .

I think both are attempting the same thing, but we need to be a lot more cautious and pro-active in detecting breaking changes.
We can decide to accept some of these breaks, but we have to know them and document them.

[apoorvdarshan]

@T-Gro Thanks for the feedback. Unlike #19322, this PR keeps the EmptyEnumerable type unchanged for backward compat — only the Seq.empty binding and the concat optimization are updated. Happy to target this for .NET 11 instead and add cross-compat test coverage for the equality/versioning scenarios you mentioned.