Add collection return support to ScanForTypes attribute

[Copilot]

ScanForTypes methods can now return Type[] or IEnumerable<Type> (no Handler needed) to get the matched types directly, or return TResponse[]/IEnumerable<TResponse> when a Handler mapping T → TResponse is provided. This feature is exclusive to ScanForTypes; GenerateServiceRegistrations is unaffected.

New capabilities

// Case 1: return matched types directly — no Handler required
[ScanForTypes(AssignableTo = typeof(IService))]
public static partial Type[] GetServiceTypes();
// Generates: return [typeof(MyService1), typeof(MyService2)];

// Case 2: map each found type via a Handler
[ScanForTypes(AssignableTo = typeof(IService), Handler = nameof(Describe))]
public static partial ServiceInfo[] GetServiceInfos();

private static ServiceInfo Describe<T>() => new(typeof(T).Name);
// Generates: return [Describe<MyService1>(), Describe<MyService2>()];

Changes

• MethodModel — added ReturnTypeIsCollection / CollectionElementTypeName fields and a GetCollectionReturnInfo() helper that detects array and IEnumerable<T> return types using SpecialType.System_Collections_Generic_IEnumerable_T.
• MethodImplementationModel — added CollectionItems (EquatableArray<string>) holding the pre-built expression strings for each collection element.
• ParseHandlerMethodModel — relaxes the "Handler required" rule when the return type is Type[]/IEnumerable<Type>; validates Handler return type matches the collection element type for mapped returns.
• FindServicesToRegister — populates CollectionItems with typeof(T) expressions (no-Handler path) or handler invocation expressions (Handler path).
• DependencyInjectionGenerator — adds GenerateCollectionSource() emitting a C# collection expression (return [...]); routes generation accordingly.
• DiagnosticDescriptors — new diagnostic DI0015 (WrongHandlerReturnTypeForCollectionReturn) when the Handler's return type doesn't match the collection element type.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Return found types as an array</issue_title>
<issue_description>In some cases I need to process found types, and using CustomHandler is not sufficient.

Acceptance Criteria:

1. When method with ScanForTypes attribute returns Type[] or IEnumerable, and Handler is not specified - we return collection of found types (use collection expressions in return).
2. When method with ScanForTypes attribute returns TResponse[] or IEnumerable, and Handler method returns TResponse (i.e. somehow maps found T type to TResponse type) - we return collection of found types mapped by Handler (use collection expressions in return).
3. If Handler return type does not correspond to collection return type - report error.
4. It is only supported for ScanForTypes attribute, not GenerateServiceRegistrations attribute.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Return found types as an array #54

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[Dreamescaper]

Update README

[Dreamescaper]

replace nester ternary with switch expression

[Dreamescaper]

split extract collection handling and custom handler to private methods

[Dreamescaper]

Update to use multiline collection expression, e.g.

                public static partial global::System.Type[] GetServiceTypes()
                {
                    return [
                        typeof(global::GeneratorTests.MyService1), 
                        typeof(global::GeneratorTests.MyService2)
                    ];
                }