Fix GenAPI omitting explicitly implemented interface events

[ViktorHofer]

Fixes #54464

GenAPI silently dropped explicit interface event implementations, causing generated reference assemblies to fail compilation with CS0535.

Root cause

Two layers:

1. SymbolExtensions.IsExplicitInterfaceImplementation only recognized IMethodSymbol and IPropertySymbol. Explicit-impl events have DeclaredAccessibility=Private and were filtered out as private members before reaching the syntax generator.
2. SyntaxGeneratorExtensions.DeclarationExt routed all events through SyntaxGenerator.CustomEventDeclaration, which has no overload that accepts an ExplicitInterfaceSpecifier, so the explicit qualification was lost even when the symbol made it through.

Changes

• SymbolExtensions: recognize IEventSymbol explicit impls. This shared helper is used by ApiCompat, ApiDiff and GenAPI, so the fix benefits all three.
• SyntaxGeneratorExtensions: new CreateExplicitInterfaceEventDeclaration helper emits EventDeclarationSyntax with ExplicitInterfaceSpecifier and empty add/remove blocks.
• CSharpAssemblyDocumentGenerator: mirror the existing method/property filter for events. An explicit-impl event whose containing interface is excluded by the symbol filter is also excluded. This preserves the invariant (already honored for methods and properties) that explicit interface implementations are part of the public API only when the underlying interface is public.
• ImplicitSymbolFilter: extend the explicit-impl accessor filter to also drop EventAdd / EventRemove accessor methods, paralleling the existing PropertyGet / PropertySet handling so accessors are not emitted twice.

Tests

• TestExplicitInterfaceEventGeneration — happy path, asserts the explicit event is emitted with the correct interface qualifier.
• TestExplicitInterfaceEventFromInternalInterfaceIsExcluded — asserts the new filter actually drops explicit events whose underlying interface is internal.

Full GenAPI test suite: 100 pass, 2 pre-existing skips, 0 failures.

[Copilot]

Pull request overview

This PR fixes GenAPI handling for explicit interface event implementations so generated reference source includes required event members and avoids CS0535 failures.

Changes:

• Recognizes explicit interface implementations for IEventSymbol.
• Emits explicit interface event declarations directly when SyntaxGenerator cannot represent them.
• Adds filtering and tests for explicit events, including excluded internal interfaces.

Show a summary per file

File	Description
src/Compatibility/Microsoft.DotNet.ApiSymbolExtensions/SymbolExtensions.cs	Extends explicit-interface detection to events.
src/Compatibility/Microsoft.DotNet.ApiSymbolExtensions/Filtering/ImplicitSymbolFilter.cs	Filters explicit event add/remove accessor methods.
src/Compatibility/GenAPI/Microsoft.DotNet.GenAPI/SyntaxGeneratorExtensions.cs	Adds direct syntax generation for explicit interface event declarations.
src/Compatibility/GenAPI/Microsoft.DotNet.GenAPI/CSharpAssemblyDocumentGenerator.cs	Filters explicit events from interfaces excluded by the symbol filter.
test/Microsoft.DotNet.GenAPI.Tests/CSharpFileBuilderTests.cs	Adds GenAPI tests for explicit interface event generation and filtering.

Copilot's findings

• Files reviewed: 5/5 changed files
• Comments generated: 1