[kotlin-client][jvm-ktor] Support nullable response types (OpenAPITools#23870)

The jvm-ktor library did not handle OpenAPI schemas that allow null
as a valid response body (e.g. anyOf: [T, {type: null}] or
allOf: [T] with nullable: true).

Two template bugs prevented this from working:

1. api.mustache used {{{returnType}}} directly, ignoring the
   isNullable flag on the response property. Now it appends ? when
   returnProperty.isNullable is true.

2. HttpResponse.kt.mustache hardcoded <T : Any> on every generic
   type parameter. This made HttpResponse<SomeType?> invalid at
   the Kotlin type-system level. Removing the : Any constraint lets
   the generated code use nullable types naturally.

Author: TheJJ

modules/openapi-generator/src/main/resources/kotlin-client/libraries/jvm-ktor/api.mustache

@@ -52,7 +52,7 @@ import com.fasterxml.jackson.databind.ObjectMapper
         {{#returnType}}
             @Suppress("UNCHECKED_CAST")
         {{/returnType}}
-        {{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}open suspend fun {{operationId}}({{#allParams}}{{{paramName}}}: {{{dataType}}}{{^required}}?{{/required}}{{^-last}}, {{/-last}}{{/allParams}}): HttpResponse<{{{returnType}}}{{^returnType}}Unit{{/returnType}}> {
+        {{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}open suspend fun {{operationId}}({{#allParams}}{{{paramName}}}: {{{dataType}}}{{^required}}?{{/required}}{{^-last}}, {{/-last}}{{/allParams}}): HttpResponse<{{{returnType}}}{{^returnType}}Unit{{/returnType}}{{#returnProperty}}{{#isNullable}}?{{/isNullable}}{{/returnProperty}}> {
 
             val localVariableAuthNames = listOf<String>({{#authMethods}}"{{name}}"{{^-last}}, {{/-last}}{{/authMethods}})
 

modules/openapi-generator/src/main/resources/kotlin-client/libraries/jvm-ktor/infrastructure/HttpResponse.kt.mustache

@@ -5,12 +5,12 @@ import io.ktor.http.isSuccess
 import io.ktor.util.reflect.TypeInfo
 import io.ktor.util.reflect.typeInfo
 
-{{#nonPublicApi}}internal {{/nonPublicApi}}{{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}open class HttpResponse<T : Any>({{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}val response: io.ktor.client.statement.HttpResponse, {{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}val provider: BodyProvider<T>) {
+{{#nonPublicApi}}internal {{/nonPublicApi}}{{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}open class HttpResponse<T>({{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}val response: io.ktor.client.statement.HttpResponse, {{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}val provider: BodyProvider<T>) {
     {{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}val status: Int = response.status.value
     {{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}val success: Boolean = response.status.isSuccess()
     {{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}val headers: Map<String, List<String>> = response.headers.mapEntries()
     {{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}suspend fun body(): T = provider.body(response)
-    {{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}suspend fun <V : Any> typedBody(type: TypeInfo): V = provider.typedBody(response, type)
+    {{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}suspend fun <V> typedBody(type: TypeInfo): V = provider.typedBody(response, type)
 
     {{#nonPublicApi}}internal {{/nonPublicApi}}{{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}companion object {
         private fun Headers.mapEntries(): Map<String, List<String>> {
@@ -21,31 +21,31 @@ import io.ktor.util.reflect.typeInfo
     }
 }
 
-{{#nonPublicApi}}internal {{/nonPublicApi}}{{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}interface BodyProvider<T : Any> {
+{{#nonPublicApi}}internal {{/nonPublicApi}}{{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}interface BodyProvider<T> {
     suspend fun body(response: io.ktor.client.statement.HttpResponse): T
-    suspend fun <V : Any> typedBody(response: io.ktor.client.statement.HttpResponse, type: TypeInfo): V
+    suspend fun <V> typedBody(response: io.ktor.client.statement.HttpResponse, type: TypeInfo): V
 }
 
-{{#nonPublicApi}}internal {{/nonPublicApi}}{{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}class TypedBodyProvider<T : Any>(private val type: TypeInfo) : BodyProvider<T> {
+{{#nonPublicApi}}internal {{/nonPublicApi}}{{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}class TypedBodyProvider<T>(private val type: TypeInfo) : BodyProvider<T> {
     @Suppress("UNCHECKED_CAST")
     override suspend fun body(response: io.ktor.client.statement.HttpResponse): T =
             response.call.body(type) as T
 
     @Suppress("UNCHECKED_CAST")
-    override suspend fun <V : Any> typedBody(response: io.ktor.client.statement.HttpResponse, type: TypeInfo): V =
+    override suspend fun <V> typedBody(response: io.ktor.client.statement.HttpResponse, type: TypeInfo): V =
             response.call.body(type) as V
 }
 
-{{#nonPublicApi}}internal {{/nonPublicApi}}{{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}class MappedBodyProvider<S : Any, T : Any>(private val provider: BodyProvider<S>, private val block: S.() -> T) : BodyProvider<T> {
+{{#nonPublicApi}}internal {{/nonPublicApi}}{{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}class MappedBodyProvider<S, T>(private val provider: BodyProvider<S>, private val block: S.() -> T) : BodyProvider<T> {
     override suspend fun body(response: io.ktor.client.statement.HttpResponse): T =
             block(provider.body(response))
 
-    override suspend fun <V : Any> typedBody(response: io.ktor.client.statement.HttpResponse, type: TypeInfo): V =
+    override suspend fun <V> typedBody(response: io.ktor.client.statement.HttpResponse, type: TypeInfo): V =
             provider.typedBody(response, type)
 }
 
-{{#nonPublicApi}}internal {{/nonPublicApi}}{{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}inline fun <reified T : Any> io.ktor.client.statement.HttpResponse.wrap(): HttpResponse<T> =
+{{#nonPublicApi}}internal {{/nonPublicApi}}{{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}inline fun <reified T> io.ktor.client.statement.HttpResponse.wrap(): HttpResponse<T> =
         HttpResponse(this, TypedBodyProvider(typeInfo<T>()))
 
-{{#nonPublicApi}}internal {{/nonPublicApi}}{{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}fun <T : Any, V : Any> HttpResponse<T>.map(block: T.() -> V): HttpResponse<V> =
+{{#nonPublicApi}}internal {{/nonPublicApi}}{{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}fun <T, V> HttpResponse<T>.map(block: T.() -> V): HttpResponse<V> =
         HttpResponse(response, MappedBodyProvider(provider, block))