Check that nullable annotations are from org.springframework.lang

Fix a few incorrect nullable annotation imports and add a checkstyle
rule to ensure they don't return.

Closes gh-28410

spring-core-test/src/main/java/org/springframework/aot/test/generator/file/DynamicFiles.java

@@ -23,7 +23,8 @@ import java.util.LinkedHashMap;
 import java.util.Map;
 import java.util.stream.Stream;
 
-import javax.annotation.Nullable;
+import org.springframework.lang.Nullable;
+
 
 /**
  * Internal class used by {@link SourceFiles} and {@link ResourceFiles} to

spring-core-test/src/main/java/org/springframework/aot/test/generator/file/ResourceFiles.java

@@ -19,7 +19,8 @@ package org.springframework.aot.test.generator.file;
 import java.util.Iterator;
 import java.util.stream.Stream;
 
-import javax.annotation.Nullable;
+import org.springframework.lang.Nullable;
+
 
 /**
  * An immutable collection of {@link ResourceFile} instances.

spring-core-test/src/main/java/org/springframework/aot/test/generator/file/SourceFile.java

@@ -18,8 +18,6 @@ package org.springframework.aot.test.generator.file;
 
 import java.io.StringReader;
 
-import javax.annotation.Nullable;
-
 import com.thoughtworks.qdox.JavaProjectBuilder;
 import com.thoughtworks.qdox.model.JavaClass;
 import com.thoughtworks.qdox.model.JavaPackage;
@@ -27,6 +25,8 @@ import com.thoughtworks.qdox.model.JavaSource;
 import org.assertj.core.api.AssertProvider;
 import org.assertj.core.util.Strings;
 
+import org.springframework.lang.Nullable;
+
 /**
  * {@link DynamicFile} that holds Java source code and provides
  * {@link SourceFileAssert} support. Usually created from an AOT generated type,

spring-core-test/src/main/java/org/springframework/aot/test/generator/file/SourceFiles.java

@@ -19,7 +19,7 @@ package org.springframework.aot.test.generator.file;
 import java.util.Iterator;
 import java.util.stream.Stream;
 
-import javax.annotation.Nullable;
+import org.springframework.lang.Nullable;
 
 /**
  * An immutable collection of {@link SourceFile} instances.