[go-server] Fix: missing quotes for string default value

[lwj5]

Default values for strings are missing double quotes in the golang server code

Fixes

• fixes #18472

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[lwj5]

On-hold. Let's merge #18533 before this so that they don't have to merge their branch

[wing328]

👌

will merge https://github.com/OpenAPITools/openapi-generator/pull/18533 shortly