Add issue links / reasons for more ExpectedWarningAttributes with tool exceptions

[jtschuster]

Adds links for a number of expected warnings in test cases that aren't produced by all 3 tools.

Adds a reason string for IL3XXX warnings that are produced by both the analyzer and Native AOT.

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[Copilot]

Pull Request Overview

This PR adds explicit reason strings (typically issue links or “NativeAOT-specific warning”) to ExpectedWarning/UnexpectedWarning attributes across many linker and analyzer test cases so that tools that don’t emit certain warnings can be better tracked and documented.

• Added a GitHub issue URL for redundant suppression warnings (IL2121/IL2071) in trimmer tests.
• Provided a “NativeAOT-specific warning” message for analyzer and Native AOT warnings (IL3002/IL3050).
• Updated behavioral tests (UnexpectedWarning) in dataflow and exponential scenarios with issue links.

Reviewed Changes

Copilot reviewed 20 out of 20 changed files in this pull request and generated no comments.

File	Description
src/tools/illink/test/Mono.Linker.Tests.Cases/Warnings/WarningSuppression/DetectRedundantSuppressionsInMembersAndTypesUsingTarget.cs	Added issue link reason to trimmer warnings
src/tools/illink/test/Mono.Linker.Tests.Cases/RequiresCapability/RequiresWithCopyAssembly.cs	Added “NativeAOT-specific warning” for method warnings
src/tools/illink/test/Mono.Linker.Tests.Cases/DataFlow/ExponentialDataFlow.cs	Marked analyzer warning as unexpected with issue link

Comments suppressed due to low confidence (3)

src/tools/illink/test/Mono.Linker.Tests.Cases/RequiresCapability/RequiresWithCopyAssembly.cs:45

• [nitpick] The literal "NativeAOT-specific warning" is duplicated across many tests. Consider defining it as a shared const (e.g. const string NativeAotWarningReason) and reusing it to reduce duplication and typos.

		[ExpectedWarning ("IL3002", "--Method--", Tool.Analyzer | Tool.NativeAot, "NativeAOT-specific warning")]

src/tools/illink/test/Mono.Linker.Tests.Cases/Warnings/WarningSuppression/DetectRedundantSuppressionsInMembersAndTypesUsingTarget.cs:40

• [nitpick] The issue link is repeated in multiple attributes throughout these test files. Extract "https://github.com/dotnet/linker/issues/1971" into a shared constant (e.g. const string LinkerIssue1971) to improve maintainability.

		[ExpectedWarning ("IL2121", "IL2071", Tool.Trimmer, "https://github.com/dotnet/linker/issues/1971")]

src/tools/illink/test/Mono.Linker.Tests.Cases/DataFlow/ExponentialDataFlow.cs:103

• [nitpick] The runtime issue URL is repeated in many test attributes here. Consider extracting "https://github.com/dotnet/runtime/issues/87596" into a constant or helper so future updates are easier.

			[ExpectedWarning ("IL2091", "'T'", Tool.Trimmer | Tool.NativeAot, "https://github.com/dotnet/runtime/issues/87596")]

[sbomer]

LGTM, thank you!