fix: parameter appending for number types (#21947)

* fix: parameter appending for number types

Updated string and number parameter handling to ensure proper conversion to string before appending.

* Update api.mustache

Author: jef

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

@@ -69,10 +69,10 @@ import com.fasterxml.jackson.databind.ObjectMapper
                         {{^isArray}}
                         {{^isString}}
                         {{^isNumber}}
-                        {{{paramName}}}?.apply { append("{{{baseName}}}", {{{paramName}}}.toString()) }
+                        {{{paramName}}}?.apply { append("{{{baseName}}}", {{{paramName}}}?.toString()) }
                         {{/isNumber}}
                         {{#isNumber}}
-                        {{{paramName}}}?.apply { append("{{{baseName}}}", {{{paramName}}}) }
+                        {{{paramName}}}?.apply { append("{{{baseName}}}", {{{paramName}}}?.toString()) }
                         {{/isNumber}}
                         {{/isString}}
                         {{#isString}}
@@ -98,10 +98,10 @@ import com.fasterxml.jackson.databind.ObjectMapper
                         {{^isArray}}
                         {{^isString}}
                         {{^isNumber}}
-                        {{{paramName}}}?.apply { it.append("{{{baseName}}}", {{{paramName}}}.toString()) }
+                        {{{paramName}}}?.apply { it.append("{{{baseName}}}", {{{paramName}}}?.toString()) }
                         {{/isNumber}}
                         {{#isNumber}}
-                        {{{paramName}}}?.apply { it.append("{{{baseName}}}", {{{paramName}}}) }
+                        {{{paramName}}}?.apply { it.append("{{{baseName}}}", {{{paramName}}}?.toString()) }
                         {{/isNumber}}
                         {{/isString}}
                         {{#isString}}