Check Custom Code When Preserve enum member underscores for backcompat

[jorgerangel-msft]

887da67 introduced back compat support for underscore enum members. However, it seems to not fully an extensible enum's custom code implementation.

In example, this enum has a custom implementation. The newly generated type removes the members that were used in custom code.

When attempting to restore the enum members with underscores, we should check the CanonicalView to validate they do not already exist in custom code.

[github-actions]

🎯 Issue Triage

When preserving enum member underscores for back-compat (introduced in commit 887da67), the C# emitter doesn't check if those members already exist in custom code, causing them to be removed and breaking custom implementations.

Category: Bug
Area: emitter:client:csharp

⚠️ No reproduction provided. A minimal repro would help diagnose this issue.

🔍 Analysis

• Affected component: http-client-csharp emitter — enum back-compat logic for underscore member names
• Root cause: The restoration logic for underscore enum members (added in 887da67) does not consult the CanonicalView to check whether those members are already defined in custom code. As a result, custom enum members are dropped from generated output.
• Impact: SDK customizations using underscore enum members (e.g., ServiceBusMinimumTlsVersion) break on regeneration.
• Fix direction: Before restoring an enum member for back-compat, check if a matching member already exists in the CanonicalView (custom code layer) and skip restoration if it does.

📝 Suggested next steps

• In the back-compat enum member restoration path, query CanonicalView for existing members with the same name before adding them to the generated output
• Add a test covering an extensible enum with custom code that includes underscore-named members

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