fix: align enum metadata generator and reflection fallback (#409)

[mkarimdev]

Fixes #409

Summary of Changes

1. Reflection Fallback Alignment (EnumMetadataRegistry.cs):

   • Updated CreateFallbackMetadata<TEnum>() to preserve declaration-order indexing during alias deduplication (.DistinctBy(x => x.value)), matching EnumMetadataGenerator's .GroupBy().Select(group => group.First()).
   • Updated secondary sorting to preserve declaration order index (x.index) following DisplayAttribute.GetOrder(). Unannotated members and equal-order members now follow declaration sequence rather than underlying constant numeric value.

2. Source Generator Attribute Matching (EnumMetadataGenerator.cs):

   • Updated AnalyzeMember attribute resolution to support derived DisplayAttribute subclasses (IsDisplayAttribute), guaranteeing parity with runtime GetCustomAttribute<DisplayAttribute>().

3. Parity Testing (EnumMetadataRegistryTests.cs & Sharprompt.Tests.csproj):

   • Added Sharprompt.SourceGenerator.csproj as an Analyzer reference in Sharprompt.Tests.csproj so compile-time generator registrations run during test execution.
   • Implemented AssertMetadataParity<TEnum>() test helper asserting identical item sequences and text selector mappings between TryGet<TEnum>() (generator) and CreateFallbackMetadata<TEnum>() (fallback).
   • Added parity test cases covering aliased enums, explicit Display.Order with unannotated members, and out-of-order declarations.

Verification

• dotnet build -c Release (0 Warnings, 0 Errors)
• dotnet test -c Release --no-build -f net10.0 (356 Passed — 5 new parity tests)
• dotnet format --verify-no-changes (0 formatting issues)