Use file urls for unpacked jars

Update `JarFileArchive` so that unpacked jars use `file:` URLs rather
than `jar:file:`. This aligns with the behavior of Spring Boot 3.1 and
allows calls to `class.getSigners()` to work again.

Fixes gh-38833

spring-boot-project/spring-boot-tools/spring-boot-loader/src/main/java/org/springframework/boot/loader/launch/JarFileArchive.java

@@ -113,7 +113,7 @@ private URL getUnpackedNestedJarUrl(JarEntry jarEntry) throws IOException {
 		if (!Files.exists(path) || Files.size(path) != jarEntry.getSize()) {
 			unpack(jarEntry, path);
 		}
-		return JarUrl.create(path.toFile());
+		return path.toUri().toURL();
 	}
 
 	private Path getTempUnpackDirectory() {

spring-boot-project/spring-boot-tools/spring-boot-loader/src/test/java/org/springframework/boot/loader/launch/JarFileArchiveTests.java

@@ -112,7 +112,8 @@ void getClassPathUrlsWhenHasUnpackCommentUnpacksAndReturnsUrls() throws Exceptio
 		assertThat(urls).hasSize(1);
 		URL url = urls.iterator().next();
 		assertThat(url).isNotEqualTo(JarUrl.create(this.file, "nested.jar"));
-		assertThat(url.toString()).startsWith("jar:file:").endsWith("/nested.jar!/");
+		// The unpack URL must be a raw file URL (see gh-38833)
+		assertThat(url.toString()).startsWith("file:").endsWith("/nested.jar").doesNotStartWith("jar:");
 	}
 
 	@Test