test(auth): Assert multiple scopes are space-separated in JWT claim (#13474)

This commit adds a test to verify that when a Service Account is
configured with multiple scopes during Self-Signed JWT authentication,
the scopes are correctly serialized into a single space-separated string
within the JWT's 'scope' claim.

This brings the Java library's test suite into alignment with the
expected auth specification. Other Google Cloud client libraries like
Go, Node.js, and Python natively assert that multiple scopes are
appropriately space-separated during their JWT generation tests.

Author: westarle

google-auth-library-java/oauth2_http/javatests/com/google/auth/oauth2/ServiceAccountCredentialsTest.java

@@ -1762,6 +1762,17 @@ void createScopes_existingAccessTokenInvalidated() throws IOException {
     assertNull(newAccessToken);
   }
 
+  @Test
+  void getRequestMetadata_withMultipleScopes_selfSignedJWT() throws IOException {
+    List<String> scopes = Arrays.asList("scope1", "scope2");
+    ServiceAccountCredentials credentials =
+        createDefaultBuilderWithKey(OAuth2Utils.privateKeyFromPkcs8(PRIVATE_KEY_PKCS8))
+            .setScopes(scopes)
+            .setUseJwtAccessWithScope(true)
+            .build();
+    verifyJwtAccess(credentials.getRequestMetadata(CALL_URI), "scope1 scope2");
+  }
+
   private void verifyJwtAccess(Map<String, List<String>> metadata, String expectedScopeClaim)
       throws IOException {
     assertNotNull(metadata);