fix(material/schematics): don't migrate unknown stylesheet formats

Fixes that we were trying to parse stylesheets that we don't support (e.g. `.sass`).

Fixes #26396.

Author: crisbeto

src/material/schematics/ng-generate/mdc-migration/rules/theming-styles.ts

@@ -8,6 +8,7 @@
 
 import {Migration, ResolvedResource} from '@angular/cdk/schematics';
 import {SchematicContext} from '@angular-devkit/schematics';
+import {extname} from 'path';
 import * as postcss from 'postcss';
 import * as scss from 'postcss-scss';
 import {ComponentMigrator, MIGRATORS, PERMANENT_MIGRATORS} from '.';
@@ -31,13 +32,21 @@ export class ThemingStylesMigration extends Migration<ComponentMigrator[], Schem
       migratedContent = migrateTypographyConfigs(migratedContent, this._namespace);
     }
 
-    this.fileSystem
-      .edit(stylesheet.filePath)
-      .remove(stylesheet.start, stylesheet.content.length)
-      .insertRight(stylesheet.start, migratedContent);
+    if (migratedContent !== stylesheet.content) {
+      this.fileSystem
+        .edit(stylesheet.filePath)
+        .remove(stylesheet.start, stylesheet.content.length)
+        .insertRight(stylesheet.start, migratedContent);
+    }
   }
 
   migrate(styles: string, filename: string): string {
+    const extension = extname(filename).toLowerCase();
+
+    if (extension && extension !== '.css' && extension !== '.scss') {
+      return styles;
+    }
+
     const processor = new postcss.Processor([
       {
         postcssPlugin: 'theming-styles-migration-plugin',

src/material/schematics/ng-update/migrations/legacy-components-v15/index.ts

@@ -19,11 +19,18 @@ import {
   MIGRATED_CORE_SYMBOLS,
 } from './constants';
 import {Migration, ResolvedResource, TargetVersion, WorkspacePath} from '@angular/cdk/schematics';
+import {extname} from 'path';
 
 export class LegacyComponentsMigration extends Migration<null> {
   enabled = this.targetVersion === TargetVersion.V15;
 
   override visitStylesheet(stylesheet: ResolvedResource): void {
+    const extension = extname(stylesheet.filePath).toLowerCase();
+
+    if (extension && extension !== '.css' && extension !== '.scss') {
+      return;
+    }
+
     let namespace: string | undefined = undefined;
     const processor = new postcss.Processor([
       {