[release/9.0-staging] Fix LINQ handling of iterator.Take(...).Last(...)

[stephentoub]

Backport of #112680 to release/9.0-staging

Customer Impact

• Customer reported
• Found internally

Code that does commonLINQOperators.Take(...).Last(), where certainLINQOperators are things like Range, Select, Where, etc., if the value passed to take is larger than the number of elements in the source, Last will throw an exception, and LastOrDefault will end up returning the default value rather than the actual last value. Essentially, the consumer can start operating on incorrect data or hit unexpected failures.

Regression

• Yes
• No

This was introduced accidentally in .NET 9 as part of a change that consolidated several internal abstractions in LINQ.

Testing

Revamped the LINQ tests to execute various combinations that lead to this, but also a bunch of other combinations to try to root out any other similar issues.

Risk

Low. The change is isolated to a single block and is now much better tested. Developers would have either been getting back erroneous results, or erroneous exceptions; it'd be difficult for someone to have taken a meaningful dependency on the bug.

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[Copilot]

PR Overview

This pull request backports a fix to the LINQ implementation so that calls to iterator.Take(...).Last(...) now correctly return the actual last element rather than a default value or throwing an exception. Key changes include:

• Adjusting a condition in the SkipTake speed‐optimized code to correctly select the proper branch when handling the last element.
• Updating internal transformation helpers and test cases to use the new CreateSources method instead of IdentityTransforms.
• Revising various test methods to improve coverage and consistency across different enumerable scenarios.

Reviewed Changes

File	Description
src/libraries/System.Linq/tests/EnumerableTests.cs	Updated transformation helpers and test cases to use CreateSources for more robust scenarios.
src/libraries/System.Linq/tests/SelectManyTests.cs	Revised parameterized tests to use a fixed loop for testing multiple ranges.
src/libraries/System.Linq/src/System/Linq/SkipTake.SpeedOpt.cs	Changed a condition in the Take method to address an issue with incorrectly retrieving the last element.
Other test files	Adjusted test expectations and minor refactorings for consistency in enumerable behavior.

Copilot reviewed 14 out of 14 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (1)

src/libraries/System.Linq/src/System/Linq/SkipTake.SpeedOpt.cs:433

• Changing the condition from '>= _minIndexInclusive' to '> _minIndexInclusive' directly affects branch selection for retrieving the last element. Please verify that this change correctly resolves the issue without introducing unintended side effects.

                    count > _minIndexInclusive)

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[artl93]

Adding approved per-email with Steve