fix(@angular/build): account for HTML base HREF for dev-server externals

When adjusting URLs to support explicit external dependencies when using Vite,
the workaround will now account for the presence of a base HREF value within
the specifier. Vite will automatically add the base HREF as a prefix to the
path when specified. This previously resulted in invalid specifiers due to
the partial removal of the Vite specific `@id` path prefix.

packages/angular/build/src/builders/dev-server/tests/behavior/build-external-dependencies_spec.ts

@@ -45,5 +45,43 @@ describeServeBuilder(executeDevServer, DEV_SERVER_BUILDER_INFO, (harness, setupT
       expect(text).toContain(`import { BehaviorSubject } from "rxjs";`);
       expect(text).toContain(`import { map } from "rxjs/operators";`);
     });
+
+    it('respects import specifiers when using baseHref with trailing slash', async () => {
+      setupTarget(harness, {
+        externalDependencies: ['rxjs', 'rxjs/operators'],
+        baseHref: '/test/',
+      });
+
+      harness.useTarget('serve', {
+        ...BASE_OPTIONS,
+      });
+
+      const { result, response } = await executeOnceAndFetch(harness, 'main.js');
+
+      expect(result?.success).toBeTrue();
+
+      const text = await response?.text();
+      expect(text).toContain(`import { BehaviorSubject } from "rxjs";`);
+      expect(text).toContain(`import { map } from "rxjs/operators";`);
+    });
+
+    it('respects import specifiers when using baseHref without trailing slash', async () => {
+      setupTarget(harness, {
+        externalDependencies: ['rxjs', 'rxjs/operators'],
+        baseHref: '/test',
+      });
+
+      harness.useTarget('serve', {
+        ...BASE_OPTIONS,
+      });
+
+      const { result, response } = await executeOnceAndFetch(harness, 'main.js');
+
+      expect(result?.success).toBeTrue();
+
+      const text = await response?.text();
+      expect(text).toContain(`import { BehaviorSubject } from "rxjs";`);
+      expect(text).toContain(`import { map } from "rxjs/operators";`);
+    });
   });
 });

packages/angular/build/src/tools/vite/id-prefix-plugin.ts

@@ -11,7 +11,7 @@ import type { Plugin } from 'vite';
 // NOTE: the implementation for this Vite plugin is roughly based on:
 // https://github.com/MilanKovacic/vite-plugin-externalize-dependencies
 
-const VITE_ID_PREFIX = '/@id/';
+const VITE_ID_PREFIX = '@id/';
 
 const escapeRegexSpecialChars = (inputString: string): string => {
   return inputString.replace(/[.*+?^${}()|[\]\\]/g, '\\$&'); // $& means the whole matched string
@@ -29,7 +29,7 @@ export function createRemoveIdPrefixPlugin(externals: string[]): Plugin {
 
       const escapedExternals = externals.map(escapeRegexSpecialChars);
       const prefixedExternalRegex = new RegExp(
-        `${VITE_ID_PREFIX}(${escapedExternals.join('|')})`,
+        `${resolvedConfig.base}${VITE_ID_PREFIX}(${escapedExternals.join('|')})`,
         'g',
       );