Fix source generator crash when parameter type uses verbatim @ keyword prefix

[Copilot]

Roslyn's FullyQualifiedFormat includes the @ verbatim escape prefix for types whose names are reserved C# keywords (e.g. global::ProjectName.Storage.@event). This @ ended up in the generated hint name, which Roslyn rejects as invalid — causing the generator to silently fail and EF Core to throw at runtime.

Root cause

AppendSanitizedTypeName stripped global:: but left @ untouched. Since @ has no CLR-level meaning (the runtime type name is just event, not @event), it must be skipped entirely — not replaced — so the generator and the runtime resolver produce identical class names.

Changes

• ProjectionExpressionClassNameGenerator.AppendSanitizedTypeName — skip @ characters in the same pass that skips global::, ensuring the generated class name (and hint/file name) never contains @
• ProjectionExpressionClassNameGeneratorTests — three new [InlineData] cases covering global::Foo.Storage.@event, bare @event, and global::Foo.@delegate
• ExtensionMethodTests — new snapshot test for an extension method whose this parameter type is @event; verified output uses Foo_EventExtensions_GetId_P0_Foo_event (no @)

Example

// Previously caused CS8785 generator failure + runtime InvalidOperationException
public static class EventExtensions
{
    [Projectable]
    public static int GetId(this @event e) => e.Id;
}