refactor(@angular/build): use async file read and warn on missing sourcemap

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: JSMike

packages/angular/build/src/tools/esbuild/javascript-transformer-worker.ts

@@ -107,14 +107,18 @@ async function transformWithBabel(
     if (externalMapMatch) {
       const mapPath = path.resolve(path.dirname(filename), externalMapMatch[1]);
       try {
-        const mapContent = fs.readFileSync(mapPath, 'utf-8');
+        const mapContent = await fs.promises.readFile(mapPath, 'utf-8');
         const inlineMap = Buffer.from(mapContent).toString('base64');
         return data.replace(
           /^\/\/# sourceMappingURL=[^\r\n]*/m,
           `//# sourceMappingURL=data:application/json;charset=utf-8;base64,${inlineMap}`,
         );
-      } catch {
+      } catch (error) {
         // Map file not readable; return data with the original external reference
+        // eslint-disable-next-line no-console
+        console.warn(
+          `Unable to inline sourcemap for '${filename}': ${error instanceof Error ? error.message : error}`,
+        );
       }
     }