Fix bug with enum customizations in TsServer codegen (#4244)

## Motivation and Context
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Fixing bug from https://github.com/smithy-lang/smithy-rs/pull/4039. It
is breaking the diff generation
([ex](https://github.com/smithy-lang/smithy-rs/actions/runs/16690158842/job/47246940590?pr=4243))
in other PRs. Can't seem to fix it in the other PR since the diff always
picks up the older broken revision, so doing a small stand-alone PR to
get this in.


## Description
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## Testing
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