[http-client-csharp] Preserve enum member underscores for backcompat

[JoshLove-msft]

Summary

• match fixed enum members against the last contract while ignoring underscores
• preserve the published member name when the underscore-insensitive match is unique
• avoid applying ambiguous underscore-insensitive matches

Validation

• npm run build
• dotnet test packages/http-client-csharp/generator
• npm run cop

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@typespec/http-client-csharp@11269

commit: 599b4a9

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[JoshLove-msft]

Validated against all 15 Azure management libraries currently using \�xact\ by removing the wrappers locally and regenerating with the locally built generator plus each library's GA baseline. All 15 generations succeeded, and released enum members retained their existing names. The only exact-related API changes were \Tls1_3\ in Event Hubs and App Service; those members are not present in their current GA baseline packages, so backcompat intentionally has no prior name to restore. --generated by Copilot