Deduplicate aliased enum values in generated metadata

[shibayan]

Summary

An enum with aliased members — multiple names sharing the same constant value, e.g.

public enum AliasEnum
{
    None = 0,
    Default = 0,
    Value = 1
}

made EnumMetadataGenerator emit a switch expression with duplicate constant patterns, breaking the consuming project's build with CS8510: The pattern is unreachable in the generated file.

Members are now deduplicated by constant value keeping the first declaration (aliases are indistinguishable at runtime anyway), for both the generated values array and the display-name switch. The reflection fallback in EnumMetadataRegistry applies the same semantics so both paths behave identically.

Note: this was also broken in v3.0.x, where EnumHelper threw ArgumentException at runtime on the duplicate dictionary key — so this has never worked, it just fails earlier (at compile time) in v3.1.

Test plan

• Verified with a standalone consumer project: the alias enum previously failed to compile (CS8510 in *.EnumMetadata.g.cs); after the fix it builds and resolves Items = [None, Value] with Default labeled None
• New unit test for the reflection fallback dedup semantics
• Full suite: 325 passed, dotnet format clean

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Fixes enum metadata generation for enums with aliased members (multiple names sharing the same constant value) by deduplicating members so generated switch expressions don’t contain duplicate constant patterns (avoiding CS8510) and aligning runtime fallback behavior with the generator’s semantics.

Changes:

• Deduplicate aliased enum members by constant value in the source generator (keep first declaration).
• Deduplicate aliased enum members in reflection fallback metadata creation.
• Add a unit test covering alias-dedup semantics for the reflection fallback.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
tests/Sharprompt.Tests/Binding/EnumMetadataRegistryTests.cs	Adds coverage to ensure aliased enum values keep the first declaration in fallback metadata.
src/Sharprompt/EnumMetadataRegistry.cs	Deduplicates reflected enum members by value to match generator behavior and avoid duplicate entries.
src/Sharprompt.SourceGenerator/EnumMetadataGenerator.cs	Deduplicates aliased enum members to prevent unreachable-pattern compiler errors in generated code.

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