[java] honor useJspecify in restclient/webclient ApiClient support class (#24055)

The restclient and webclient ApiClient support classes hardcoded
'import {javaxPackage}.annotation.Nullable;' regardless of useJspecify, so
generated clients used org.jspecify everywhere except ApiClient, which kept
jakarta/javax. Guard the import on useJspecify; @nullable is used as a simple
name so only the import changes. Regenerated the two affected jspecify samples.

Author: austek

modules/openapi-generator/src/main/resources/Java/libraries/restclient/ApiClient.mustache

@@ -79,7 +79,12 @@ import java.util.Map.Entry;
 import java.util.TimeZone;
 import java.util.function.Supplier;
 
+{{#useJspecify}}
+import org.jspecify.annotations.Nullable;
+{{/useJspecify}}
+{{^useJspecify}}
 import {{javaxPackage}}.annotation.Nullable;
+{{/useJspecify}}
 
 {{#jsr310}}
 import java.time.OffsetDateTime;

modules/openapi-generator/src/main/resources/Java/libraries/webclient/ApiClient.mustache

@@ -82,7 +82,12 @@ import java.util.Map;
 import java.util.Map.Entry;
 import java.util.TimeZone;
 
+{{#useJspecify}}
+import org.jspecify.annotations.Nullable;
+{{/useJspecify}}
+{{^useJspecify}}
 import {{javaxPackage}}.annotation.Nullable;
+{{/useJspecify}}
 
 {{#jsr310}}
 import java.time.OffsetDateTime;

samples/client/petstore/java/restclient-springBoot4-jackson3-jspecify/src/main/java/org/openapitools/client/ApiClient.java

@@ -53,7 +53,7 @@
 import java.util.TimeZone;
 import java.util.function.Supplier;
 
-import jakarta.annotation.Nullable;
+import org.jspecify.annotations.Nullable;
 
 import java.time.OffsetDateTime;
 

samples/client/petstore/java/webclient-springBoot4-jackson3-jspecify/src/main/java/org/openapitools/client/ApiClient.java

@@ -70,7 +70,7 @@
 import java.util.Map.Entry;
 import java.util.TimeZone;
 
-import jakarta.annotation.Nullable;
+import org.jspecify.annotations.Nullable;
 
 import java.time.OffsetDateTime;