fix: move import statements to top in generated web component imports file (#23817) (CP: 25.0) (#23820)

This PR cherry-picks changes from the original PR #23817 to branch 25.0.
---
#### Original PR description
> `AbstractUpdateImports.process()` already reorders `import` lines to
> the top for `generatedFlowImports`, but the same sorting was not
> applied to `generatedFlowWebComponentImports`. This caused interleaved
> `import` and non-import lines (e.g. `injectGlobalWebcomponentCss()`
> calls mixed with `import` statements) in the web component output.
>
> Extract a reusable `moveImportsToTop()` method and apply it both in
> `process()` for main imports and in `mergeWebComponentOutputLines()`
> after merging and deduplicating the web component sources.
>
> Related to #23689 (comment)

Author: mcollovati

flow-server/src/main/java/com/vaadin/flow/server/frontend/AbstractUpdateImports.java

@@ -267,7 +267,18 @@ private List<String> mergeWebComponentOutputLines(
                 Collections.emptyList()));
         merged.addAll(outputFiles.getOrDefault(generatedFlowImports,
                 Collections.emptyList()));
-        return merged.stream().distinct().toList();
+        List<String> result = new ArrayList<>(
+                merged.stream().distinct().toList());
+        moveImportsToTop(result);
+        return result;
+    }
+
+    // Move all import lines to the top, before any non-import lines
+    private static void moveImportsToTop(List<String> lines) {
+        List<String> imports = new ArrayList<>(lines);
+        imports.removeIf(line -> !line.startsWith("import "));
+        lines.removeIf(line -> line.startsWith("import "));
+        lines.addAll(0, imports);
     }
 
     private void writeWebComponentImports(List<String> lines) {
@@ -441,11 +452,7 @@ private Map<File, List<String>> process(Map<ChunkInfo, List<CssData>> css,
                     getCssLines(webComponentCssData, cssLineOffset));
         }
 
-        // Move all imports to the top
-        List<String> copy = new ArrayList<>(mainLines);
-        copy.removeIf(line -> !line.startsWith("import "));
-        mainLines.removeIf(line -> line.startsWith("import "));
-        mainLines.addAll(0, copy);
+        moveImportsToTop(mainLines);
 
         mainLines.addAll(chunkLoader);
         mainLines.add("window.Vaadin = window.Vaadin || {};");

flow-server/src/test/java/com/vaadin/flow/server/frontend/AbstractUpdateImportsTest.java

@@ -48,10 +48,12 @@
 import com.vaadin.experimental.FeatureFlags;
 import com.vaadin.flow.component.Component;
 import com.vaadin.flow.component.UI;
+import com.vaadin.flow.component.WebComponentExporter;
 import com.vaadin.flow.component.dependency.CssImport;
 import com.vaadin.flow.component.dependency.JavaScript;
 import com.vaadin.flow.component.dependency.JsModule;
 import com.vaadin.flow.component.page.AppShellConfigurator;
+import com.vaadin.flow.component.webcomponent.WebComponent;
 import com.vaadin.flow.router.Route;
 import com.vaadin.flow.server.Constants;
 import com.vaadin.flow.server.LoadDependenciesOnStartup;
@@ -108,6 +110,19 @@ public static class FooCssImport2 extends Component {
     public static class ThemeCssImport implements AppShellConfigurator {
     }
 
+    @CssImport("./foo.css")
+    public static class CssImportExporter
+            extends WebComponentExporter<FooCssImport> {
+        public CssImportExporter() {
+            super("css-import-exporter");
+        }
+
+        @Override
+        public void configureInstance(WebComponent<FooCssImport> webComponent,
+                FooCssImport component) {
+        }
+    }
+
     protected File tmpRoot;
     protected File frontendDirectory;
     protected File nodeModulesPath;
@@ -876,4 +891,45 @@ private void assertImportOrder(String... imports) {
         }
     }
 
+    @Test
+    public void generatedFlowImports_importsAreOnTopBeforeOtherInstructions() {
+        updater.run();
+
+        List<String> lines = updater.getOutput()
+                .get(updater.generatedFlowImports);
+        assertImportsBeforeNonImportLines(lines);
+    }
+
+    @Test
+    public void generatedFlowWebComponentImports_importsAreOnTopBeforeOtherInstructions()
+            throws Exception {
+        Class<?>[] testClasses = { CssImportExporter.class, FooCssImport.class,
+                UI.class, AllEagerAppConf.class };
+        ClassFinder classFinder = getClassFinder(testClasses);
+        updater = new UpdateImports(getScanner(classFinder), options);
+        updater.run();
+
+        List<String> lines = updater.webComponentImports;
+        Assert.assertNotNull("Web component imports should have been generated",
+                lines);
+        assertImportsBeforeNonImportLines(lines);
+    }
+
+    private void assertImportsBeforeNonImportLines(List<String> lines) {
+        boolean seenNonImport = false;
+        for (String line : lines) {
+            if (line.isBlank()) {
+                continue;
+            }
+            if (!line.startsWith("import ")) {
+                seenNonImport = true;
+            } else if (seenNonImport) {
+                Assert.fail("Import line found after non-import line. "
+                        + "All import lines should be at the top.\n"
+                        + "Offending line: " + line + "\n" + "Full output:\n"
+                        + String.join("\n", lines));
+            }
+        }
+    }
+
 }