Add back MinInteger/MaxInteger paths for IEnumerables in LINQ

[MihaZupan]

Fixes #130047 - perf regression from #128306 for IEnumerable inputs where we can't extract the span, and the T is an integer type. #128306 removed the loop that'll go over the IEnumerable and compare results via < instead of going through IComparer. This PR adds that logic back.

MihuBot/runtime-utils#2066

Method	Toolchain	input	Mean	Error	Ratio	Allocated	Alloc Ratio
Min	Main	IEnumerable	102.14 ns	0.418 ns	1.00	32 B	1.00
Min	PR	IEnumerable	44.23 ns	1.275 ns	0.43	-	0.00
Max	Main	IEnumerable	107.91 ns	0.047 ns	1.00	32 B	1.00
Max	PR	IEnumerable	56.38 ns	0.198 ns	0.52	-	0.00

The intention before merging that PR was to avoid such regressions, but looks like my benchmark in #128306 (comment) didn't catch it EgorBot/Benchmarks#208 (comment).

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[Copilot]

Pull request overview

This PR reintroduces specialized Min/Max fast paths in System.Linq.Enumerable for non-span IEnumerable<T> inputs when T is an integer type and the caller uses the default comparer (or null comparer), avoiding per-element Comparer<T>.Default.Compare overhead.

Changes:

• Added MinIntegerEnumerator<T> / MaxIntegerEnumerator<T> helpers that compute min/max via direct < / > comparisons for IBinaryInteger<T> types.
• Updated the generic Min<TSource>(IEnumerable<TSource>, IComparer<TSource>?) and Max<TSource>(...) implementations to detect common integer types and route to the new enumerator helpers when the comparer is default or null.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Linq/src/System/Linq/Min.cs	Adds an integer enumerator-based min helper and routes the generic Min<TSource> to it for non-span integer sequences with default/null comparer.
src/libraries/System.Linq/src/System/Linq/Max.cs	Adds an integer enumerator-based max helper and routes the generic Max<TSource> to it for non-span integer sequences with default/null comparer.