fix!: Remove distinction of enum and flags-enum for ShapeTag

[google-labs-jules]

This change removes the distinction between enum and flags-enum for the ShapeTag generation, ensuring that both regular and [Flags] enums are prefixed with "enum:" in ShapeTags. It also cleans up the unused HasFlagsAttribute method from ZeroSerializerGenerator.cs, and updates corresponding assertions in ShapeTagAndHashTests.cs.

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PR created automatically by Jules for task 10708314756535983563 started by @sator-imaging

[google-labs-jules]

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