fix(android,auth): return correct error code for linkWithCredential provider-already-linked on Android (#8245)

Author: pr-Mais

packages/firebase_auth/firebase_auth/android/src/main/java/io/flutter/plugins/firebase/auth/FlutterFirebaseAuthPlugin.java

@@ -806,7 +806,21 @@ private Task<Map<String, Object>> linkUserWithCredential(Map<String, Object> arg
             throw FlutterFirebaseAuthPluginException.invalidCredential();
           }
 
-          AuthResult authResult = Tasks.await(firebaseUser.linkWithCredential(credential));
+          AuthResult authResult;
+
+          try {
+            authResult = Tasks.await(firebaseUser.linkWithCredential(credential));
+          } catch (Exception exception) {
+            String message = exception.getMessage();
+
+            if (message != null
+                && message.contains("User has already been linked to the given provider.")) {
+              throw FlutterFirebaseAuthPluginException.alreadyLinkedProvider();
+            }
+
+            throw exception;
+          }
+
           return parseAuthResult(authResult);
         });
   }

packages/firebase_auth/firebase_auth/android/src/main/java/io/flutter/plugins/firebase/auth/FlutterFirebaseAuthPluginException.java

@@ -75,6 +75,11 @@ static FlutterFirebaseAuthPluginException noSuchProvider() {
         "NO_SUCH_PROVIDER", "User was not linked to an account with the given provider.");
   }
 
+  static FlutterFirebaseAuthPluginException alreadyLinkedProvider() {
+    return new FlutterFirebaseAuthPluginException(
+        "PROVIDER_ALREADY_LINKED", "User has already been linked to the given provider.");
+  }
+
   public String getCode() {
     return code.toLowerCase(Locale.ROOT).replace("error_", "").replace("_", "-");
   }