Update Enumerable.Sequence based on post-merge feedback

[stephentoub]

• Use T.IsPositive/IsZero
• Put zero check first
• Throw for NaN
• Rename a parameter to clarify its purpose
• Add some comments

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[Copilot]

Pull Request Overview

This PR enhances the Sequence and AsyncSequence APIs to validate NaN, handle a zero step specially, renames a test method, and fleshes out the ReferenceAddable stub with the new INumber<T> members.

• Add T.IsNaN checks before range and step logic
• Introduce explicit handling when step == 0 to either repeat once or throw
• Rename tests to InvalidArguments_Throws and implement new INumber members in ReferenceAddable

Reviewed Changes

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

File	Description
src/libraries/System.Linq/tests/SequenceTests.cs	Renamed test and added NaN-argument assertions
src/libraries/System.Linq/src/System/Linq/Sequence.cs	Added NaN and zero-step checks and refactored iterator locals
src/libraries/System.Linq.AsyncEnumerable/tests/SequenceTests.cs	Renamed test and added NaN-argument assertions
src/libraries/System.Linq.AsyncEnumerable/src/System/Linq/Sequence.cs	Added NaN and zero-step checks and refactored iterator locals

Comments suppressed due to low confidence (2)

src/libraries/System.Linq/tests/SequenceTests.cs:23

• Add tests for the zero-step behavior: when step == T.Zero and start == endInclusive, the sequence should yield exactly one element, and when start != endInclusive, it should throw ArgumentOutOfRangeException.

            AssertExtensions.Throws<ArgumentOutOfRangeException>("step", () => Enumerable.Sequence(1.0f, 1.0f, float.NaN));

src/libraries/System.Linq.AsyncEnumerable/tests/SequenceTests.cs:24

• Similarly, add async tests covering the step == T.Zero cases: one element when start == endInclusive and throwing when start != endInclusive.

            AssertExtensions.Throws<ArgumentOutOfRangeException>("step", () => AsyncEnumerable.Sequence(1.0f, 1.0f, float.NaN));

[stephentoub]

/ba-g infra failures