fix: Install node when npm/npx not found in getNpmCliToolExecutable

When npm/npx couldn't be found in any location (project, global, or
alternative dir), getNpmCliToolExecutable would return an empty list.
Flags like --no-update-notifier were then added to this empty list,
resulting in a malformed command that tried to execute the flag as
the program name.

Added a fallback that triggers node installation when npm/npx is not
found and an alternativeDirGetter is available. After installation,
uses the paths from activeNodeInstallation which correctly point to
the version-specific node installation directory (e.g., node-v24.10.0)
rather than the old non-versioned 'node' directory.

Author: Artur-

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

@@ -885,6 +885,30 @@ private List<String> getNpmCliToolExecutable(BuildTool cliTool,
             forceAlternativeNode = true;
         }
 
+        // If npm/npx still not found, install node (which includes npm/npx)
+        if (returnCommand.isEmpty() && alternativeDirGetter != null
+                && (cliTool.equals(BuildTool.NPM)
+                        || cliTool.equals(BuildTool.NPX))) {
+            forceAlternativeNodeExecutable();
+            // After installation, activeNodeInstallation should be populated
+            // with the correct paths for version-specific node installation
+            ActiveNodeInstallation installed = activeNodeInstallation;
+            if (installed != null && installed.npmCliScript() != null) {
+                returnCommand = new ArrayList<>();
+                returnCommand.add(installed.nodeExecutable());
+                if (cliTool.equals(BuildTool.NPM)) {
+                    returnCommand.add(installed.npmCliScript());
+                } else {
+                    // NPX is in the same directory as npm, just different
+                    // filename
+                    File npmCliFile = new File(installed.npmCliScript());
+                    File npxCliFile = new File(npmCliFile.getParentFile(),
+                            "npx-cli.js");
+                    returnCommand.add(npxCliFile.getAbsolutePath());
+                }
+            }
+        }
+
         if (flags.length > 0) {
             returnCommand = new ArrayList<>(returnCommand);
             Collections.addAll(returnCommand, flags);