fix(kotlin): forbid enum case names equal to the enum type name

[schani]

Bug

Kotlin enum-typed output could generate an invalid enum constant with the
same name as its enclosing enum class, e.g.:

enum class Category(val value: String) {
    Category(" "), // invalid: same name as the enum class itself
    Empty(" ");
    ...
}

This happens when a JSON string value with no identifier-safe characters
(e.g. whitespace-only or a NUL byte) legalizes to an empty name. quicktype's
naming falls back to Empty for such values, and when two case values
collide on that fallback, ConvenienceRenderer.makeNameForEnumCase proposes
an "alternative" name of the form ${enumCombinedName}_${caseName} to
disambiguate them. Kotlin's name-styling strips all non-identifier
characters from that alternative, so a value like "Category_ " collapses
down to just "Category" — colliding, silently, with the enum type's own
name.

Root cause

KotlinRenderer.forbiddenForEnumCases (in
packages/quicktype-core/src/language/Kotlin/KotlinRenderer.ts) didn't
forbid the enum's own type name from being reused as a case name, so nothing
caught this second-order collision.

Fix

KotlinRenderer.forbiddenForEnumCases now includes the enum's own Name in
its forbidden-names list, forcing the namer to keep looking for a
non-colliding alternative instead of silently reusing the enum's own name.
This follows the same pattern already used by CSharpRenderer for object
properties (forbiddenForObjectProperties forbids the class's own name).

Test coverage

Added test/inputs/json/priority/kotlin-enum-class-case-collision.json, a
JSON fixture (runs for every language, per priority/ fixture conventions)
with repeated whitespace-only and empty-string values for a category
property, enough to trigger quicktype's string-to-enum inference and
reproduce the exact class-name/case-name collision from the issue.

Verification

• npm run build passes.
• npm run test:unit passes (163/163).
• Manually reproduced the bug on unfixed master via
  node dist/index.js --src <repro.json> --src-lang json --lang kotlin --framework jackson --top-level SomeClass,
  confirmed the generated enum class Category { Category(...), ... }
  collision, then confirmed after the fix the case is renamed (e.g. to
  Purple) instead of colliding with the class name.
• Ran the new fixture through Jackson, Klaxon, and kotlinx-flavored Kotlin
  generation manually — all produce non-colliding case names.
• QUICKTEST=true FIXTURE=python script/test passes (62/62), confirming the
  new shared JSON fixture doesn't break other languages.
• The Kotlin fixture test itself (which compiles/runs the generated code)
  was not run locally because kotlinc/kotlin are not installed in this
  environment; CI will exercise FIXTURE=kotlin and the other Kotlin
  variants (kotlin-jackson, kotlinx) against the new fixture.

Fixes #1815.

🤖 Generated with Claude Code