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