perf: use HashSet<string> for NoWarn code lookups

[mfogliatto-dev-agent]

Summary

Replaces IEnumerable<string> with HashSet<string> for NoWarn codes at the provider level, converting O(n) linear LINQ Contains lookups to O(1) HashSet lookups in ReferenceEvaluationContextFactory.Create.

Changes

File	Change
INoWarnAssembliesProvider.cs	Dictionary value type IEnumerable<string> → HashSet<string>
NoWarnAssembliesProvider.cs	Produces HashSet<string> instead of string[]
ProjectReferenceInfo.cs	NoWarn property type IEnumerable<string> → HashSet<string>
MSBuildProjectMetadataProvider.cs	Wraps split results in HashSet<string>

How it works

The optimization is applied at the provider level — where NoWarn codes are first materialized from config strings. This means:

• Roslyn path: NoWarnAssembliesProvider.GetNoWarnByAssembly() now returns Dictionary<string, HashSet<string>>. The HashSet values flow through ReferenceCopAnalyzer.AnalyzeCompilation into ReferenceEvaluationContextFactory.Create, where LINQ's Enumerable.Contains() detects the underlying ICollection<T> and dispatches to HashSet<T>.Contains() — O(1).
• MSBuild path: MSBuildProjectMetadataProvider produces ProjectReferenceInfo with HashSet<string> NoWarn codes, achieving the same O(1) lookup.

The factory's parameter type (IEnumerable<string>) is intentionally unchanged to minimize API surface impact — the performance gain comes from the concrete HashSet passed in at runtime.

Testing

• All existing tests pass (Roslyn: 11/11, MSBuild: 14/14)
• NoWarnAssembliesProviderTests validates the provider behavior with the new HashSet return type

Fixes #71