Check custom code when preserving enum member underscores for backcompat

[Copilot]

The underscore back-compat pass restores previously-shipped underscore names (e.g. Tls1_0) when the current spec normalizes them (e.g. Tls10). For extensible enums whose custom code re-adds the underscore member, the restoration renamed the generated member back to the underscore form — colliding with the custom member and removing the generated symbol it referenced.

Example custom code that broke:

public partial struct ServiceBusMinimumTlsVersion
{
    // references the generated `Tls10`; back-compat renamed it to `Tls1_0`,
    // producing a duplicate and dropping `Tls10`
    public static ServiceBusMinimumTlsVersion Tls1_0 { get; } = Tls10;
}

Changes

• EnumProvider.GetBackCompatibleName: new optional customMemberNames argument; skips the underscore restoration when the restored name already exists in custom code, keeping the generated name.
• ExtensibleEnumProvider.BuildEnumValues: collects custom member names from CustomCodeView.Properties (case-insensitive) and threads them through. Uses CustomCodeView rather than CanonicalView to avoid the CanonicalView → Properties → EnumValues → BuildEnumValues recursion cycle; it contributes the same custom members.
• Tests: adds BackCompat_ExtensibleEnumCustomCodeTakesPrecedenceOverPreservedUnderscores with (Custom)/(Last) TestData; existing BackCompat_ExtensibleEnumUnderscoresPreserved remains as the no-custom-code case.

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commit: 0f7dcfd