Decode kotlin.Unit suspend return values to Unit.INSTANCE instead of null

[velo]

Summary

• `feign-kotlin`'s `CoroutineFeignTest` never actually ran under surefire 3.5.6 (confirmed on a pristine clone of master: `Tests run: 0` for that module). Surefire 3.6.0 (see build(deps): Bump maven-surefire-plugin.version from 3.5.6 to 3.6.0 #3556) fixes JUnit Platform test discovery for this class and in doing so exposed a real bug.
• `InvocationContext.isVoidType()` treats `kotlin.Unit` the same as Java `void`/`Void` and returns `null` for it. But Kotlin's `Unit` is non-nullable, so a suspend function declared to return `Unit` needs the singleton `Unit.INSTANCE`, not `null`.
• Adds a reflective resolution (feign-core has no compile-time dependency on kotlin-stdlib) that returns `Unit.INSTANCE` specifically for `kotlin.Unit` return types, leaving `void`/`Void` behavior unchanged.

Test plan

• `CoroutineFeignTest` (6/6) passes with surefire 3.6.0
• `feign-core`'s full suite (729 tests) unaffected
• `mvn verify` on `core`,`kotlin` (including japicmp binary-compat check) passes