[kotlin] register service-loaded Jackson modules with useJackson3
The Jackson 2 branch of Serializer.kt.mustache calls findAndRegisterModules(), so modules published via META-INF/services are registered. The Jackson 3 branch added in #24709 only calls addModule(kotlinModule()), so they are silently dropped.
Measured with jackson-databind 3.1.5 and jackson-databind-nullable:0.2.11, which ships that service file:
JsonMapper.builder().build()->{"name":{"present":true,"undefined":false}}.findAndAddModules().build()->{"name":"x"}
Adding a module explicitly and by discovery registers it once, so addModule(kotlinModule()) stays.
Both jackson3 samples regenerated; ./gradlew compileKotlin on kotlin-jackson3 passes. The new test fails on master.
Fixes #24859
/cc @jimschubert @stefankoppier
Summary by cubic
Registers service-loaded Jackson modules when generating Kotlin clients with useJackson3, fixing #24859.
The Jackson 3 branch of Serializer.kt only added the Kotlin module, so third-party modules published via META-INF/services were silently dropped. It now calls findAndAddModules() to match Jackson 2 behavior. Explicit module registration is still deduplicated, so addModule(kotlinModule()) remains.