fix: allow Kotlin generators to emit fields named '_'

[cnaples79]

Summary

• sanitize Kotlin identifier generation so schema fields like _ map to legal Kotlin members
• propagate the sanitized names through Kotlin and Kotlin2 generators
• add regression tests that compile underscore fields and check JsonProperty metadata

Rationale

• fixes Kotlin compilation failures when GraphQL schemas expose members named _

Changes

• add a shared Kotlin identifier sanitizer and update reserved keyword filtering to keep _
• apply sanitized identifiers across Kotlin/Kotlin2 data, interface, constants, entity, and client generation
• add regression tests for both generator paths covering underscore fields

Fixes #848

[sjohnr]

This looks good, thanks @cnaples79! One minor comment below.

[cnaples79]

Thanks for spotting that @sjohnr I wired the helper into sanitizeKotlinIdentifier so it now delegates to KotlinReservedKeywordSanitizer (while keeping the special-case for _).

[paulbakker]

I'm wondering if we should just remove this completely now that it's basically a no-op.

[paulbakker]

@jiholee17 do you want to take a look at removing this filter after we merge this?

[jiholee17]

sure, I can take a look

[paulbakker]

@cnaples79 There are test failures, can you look into those?

[sjohnr]

@cnaples79 just checking in if you were able to look at the test failures? It looks as though fields are now generating with a _ prefix, so the latest changes might not be correct yet.