[kotlin] Add = null default for optional parameters in jvm-ktor and javalin templates

Fixes #23100

Several Kotlin templates made optional parameters nullable (Type?) but
did not assign = null as a default value, forcing callers to explicitly
pass null for every optional parameter instead of omitting them.

Updated templates:
- kotlin-client/libraries/jvm-ktor/api.mustache
- kotlin-server/libraries/javalin5/service.mustache
- kotlin-server/libraries/javalin5/serviceImpl.mustache
- kotlin-server/libraries/javalin6/service.mustache
- kotlin-server/libraries/javalin6/serviceImpl.mustache

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: halotukozak

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}}{{#defaultValue}} = {{{defaultValue}}}{{/defaultValue}}{{/required}}{{^required}}?{{#defaultValue}} = {{{defaultValue}}}{{/defaultValue}}{{^defaultValue}} = null{{/defaultValue}}{{/required}}{{^-last}}, {{/-last}}{{/allParams}}): HttpResponse<{{{returnType}}}{{^returnType}}Unit{{/returnType}}> {
 
             val localVariableAuthNames = listOf<String>({{#authMethods}}"{{name}}"{{^-last}}, {{/-last}}{{/authMethods}})
 

modules/openapi-generator/src/main/resources/kotlin-server/libraries/javalin5/service.mustache

@@ -27,7 +27,7 @@ interface {{classname}}Service {
     {{/externalDocs}}
      * @see {{classname}}#{{operationId}}
      */
-    {{#reactive}}{{^isArray}}suspend {{/isArray}}{{/reactive}}fun {{operationId}}({{#allParams}}{{{paramName}}}: {{^isBodyParam}}{{>optionalDataType}}{{/isBodyParam}}{{#isBodyParam}}{{^reactive}}{{>optionalDataType}}{{/reactive}}{{#reactive}}{{^isArray}}{{>optionalDataType}}{{/isArray}}{{#isArray}}Flow<{{{baseType}}}>{{/isArray}}{{/reactive}}{{/isBodyParam}}{{^-last}}, {{/-last}}{{/allParams}}): {{>returnTypes}}
+    {{#reactive}}{{^isArray}}suspend {{/isArray}}{{/reactive}}fun {{operationId}}({{#allParams}}{{{paramName}}}: {{^isBodyParam}}{{>optionalDataType}}{{/isBodyParam}}{{#isBodyParam}}{{^reactive}}{{>optionalDataType}}{{/reactive}}{{#reactive}}{{^isArray}}{{>optionalDataType}}{{/isArray}}{{#isArray}}Flow<{{{baseType}}}>{{/isArray}}{{/reactive}}{{/isBodyParam}}{{^required}}{{^defaultValue}} = null{{/defaultValue}}{{#defaultValue}} = {{{defaultValue}}}{{/defaultValue}}{{/required}}{{^-last}}, {{/-last}}{{/allParams}}): {{>returnTypes}}
 {{/operation}}
 }
 {{/operations}}

modules/openapi-generator/src/main/resources/kotlin-server/libraries/javalin5/serviceImpl.mustache

@@ -10,7 +10,7 @@ import kotlinx.coroutines.flow.Flow
 class {{classname}}ServiceImpl : {{classname}}Service {
 {{#operation}}
 
-    override {{#reactive}}{{^isArray}}suspend {{/isArray}}{{/reactive}}fun {{operationId}}({{#allParams}}{{paramName}}: {{^isBodyParam}}{{>optionalDataType}}{{/isBodyParam}}{{#isBodyParam}}{{^reactive}}{{>optionalDataType}}{{/reactive}}{{#reactive}}{{^isArray}}{{>optionalDataType}}{{/isArray}}{{#isArray}}Flow<{{{baseType}}}>{{/isArray}}{{/reactive}}{{/isBodyParam}}{{^-last}}, {{/-last}}{{/allParams}}): {{>returnTypes}} {
+    override {{#reactive}}{{^isArray}}suspend {{/isArray}}{{/reactive}}fun {{operationId}}({{#allParams}}{{paramName}}: {{^isBodyParam}}{{>optionalDataType}}{{/isBodyParam}}{{#isBodyParam}}{{^reactive}}{{>optionalDataType}}{{/reactive}}{{#reactive}}{{^isArray}}{{>optionalDataType}}{{/isArray}}{{#isArray}}Flow<{{{baseType}}}>{{/isArray}}{{/reactive}}{{/isBodyParam}}{{^required}}{{^defaultValue}} = null{{/defaultValue}}{{#defaultValue}} = {{{defaultValue}}}{{/defaultValue}}{{/required}}{{^-last}}, {{/-last}}{{/allParams}}): {{>returnTypes}} {
         TODO("Implement me")
     }
 {{/operation}}

modules/openapi-generator/src/main/resources/kotlin-server/libraries/javalin6/service.mustache

@@ -29,7 +29,7 @@ interface {{classname}}Service {
     {{/externalDocs}}
      * @see {{classname}}#{{operationId}}
      */
-    {{#reactive}}{{^isArray}}suspend {{/isArray}}{{/reactive}}fun {{operationId}}({{#allParams}}{{{paramName}}}: {{^isBodyParam}}{{>optionalDataType}}{{/isBodyParam}}{{#isBodyParam}}{{^reactive}}{{>optionalDataType}}{{/reactive}}{{#reactive}}{{^isArray}}{{>optionalDataType}}{{/isArray}}{{#isArray}}Flow<{{{baseType}}}>{{/isArray}}{{/reactive}}{{/isBodyParam}}{{^-last}}, {{/-last}}{{/allParams}}{{#hasParams}}, {{/hasParams}}ctx: Context): {{>returnTypes}}
+    {{#reactive}}{{^isArray}}suspend {{/isArray}}{{/reactive}}fun {{operationId}}({{#allParams}}{{{paramName}}}: {{^isBodyParam}}{{>optionalDataType}}{{/isBodyParam}}{{#isBodyParam}}{{^reactive}}{{>optionalDataType}}{{/reactive}}{{#reactive}}{{^isArray}}{{>optionalDataType}}{{/isArray}}{{#isArray}}Flow<{{{baseType}}}>{{/isArray}}{{/reactive}}{{/isBodyParam}}{{^required}}{{^defaultValue}} = null{{/defaultValue}}{{#defaultValue}} = {{{defaultValue}}}{{/defaultValue}}{{/required}}{{^-last}}, {{/-last}}{{/allParams}}{{#hasParams}}, {{/hasParams}}ctx: Context): {{>returnTypes}}
 {{/operation}}
 }
 {{/operations}}

modules/openapi-generator/src/main/resources/kotlin-server/libraries/javalin6/serviceImpl.mustache

@@ -11,7 +11,7 @@ import kotlinx.coroutines.flow.Flow
 class {{classname}}ServiceImpl : {{classname}}Service {
 {{#operation}}
 
-    override {{#reactive}}{{^isArray}}suspend {{/isArray}}{{/reactive}}fun {{operationId}}({{#allParams}}{{paramName}}: {{^isBodyParam}}{{>optionalDataType}}{{/isBodyParam}}{{#isBodyParam}}{{^reactive}}{{>optionalDataType}}{{/reactive}}{{#reactive}}{{^isArray}}{{>optionalDataType}}{{/isArray}}{{#isArray}}Flow<{{{baseType}}}>{{/isArray}}{{/reactive}}{{/isBodyParam}}{{^-last}}, {{/-last}}{{/allParams}}{{#hasParams}}, {{/hasParams}}ctx: Context): {{>returnTypes}} {
+    override {{#reactive}}{{^isArray}}suspend {{/isArray}}{{/reactive}}fun {{operationId}}({{#allParams}}{{paramName}}: {{^isBodyParam}}{{>optionalDataType}}{{/isBodyParam}}{{#isBodyParam}}{{^reactive}}{{>optionalDataType}}{{/reactive}}{{#reactive}}{{^isArray}}{{>optionalDataType}}{{/isArray}}{{#isArray}}Flow<{{{baseType}}}>{{/isArray}}{{/reactive}}{{/isBodyParam}}{{^required}}{{^defaultValue}} = null{{/defaultValue}}{{#defaultValue}} = {{{defaultValue}}}{{/defaultValue}}{{/required}}{{^-last}}, {{/-last}}{{/allParams}}{{#hasParams}}, {{/hasParams}}ctx: Context): {{>returnTypes}} {
         TODO("Implement me")
     }
 {{/operation}}