fix(@angular/ssr): add validation to prevent use of provideServerRoutesConfig in browser context

Introduced an error check to ensure that 'provideServerRoutesConfig' is not utilized in the browser part of the application. This helps avoid unintended behavior.

(cherry picked from commit 58a648a)

Author: alan-agius4

packages/angular/ssr/src/global.d.ts

@@ -7,3 +7,15 @@
  */
 
 declare const ngDevMode: boolean | undefined;
+
+/**
+ * Indicates whether the application is operating in server-rendering mode.
+ *
+ * `ngServerMode` is a global flag set by Angular's server-side rendering mechanisms,
+ * typically configured by `provideServerRendering` and `platformServer` during runtime.
+ *
+ * @remarks
+ * - **Internal Angular Flag**: This is an *internal* Angular flag (not a public API), avoid relying on it in application code.
+ * - **Avoid Direct Use**: This variable is intended for runtime configuration; it should not be accessed directly in application code.
+ */
+declare const ngServerMode: boolean | undefined;

packages/angular/ssr/src/routes/route-config.ts

@@ -169,6 +169,12 @@ export const SERVER_ROUTES_CONFIG = new InjectionToken<ServerRoute[]>('SERVER_RO
  * @developerPreview
  */
 export function provideServerRoutesConfig(routes: ServerRoute[]): EnvironmentProviders {
+  if (typeof ngServerMode === 'undefined' || !ngServerMode) {
+    throw new Error(
+      `The 'provideServerRoutesConfig' function should not be invoked within the browser portion of the application.`,
+    );
+  }
+
   return makeEnvironmentProviders([
     {
       provide: SERVER_ROUTES_CONFIG,