fix(auth): simplify provider ID validation and remove unused imports

Author: macastelaz

src/main/java/com/google/firebase/auth/OidcProviderConfig.java

@@ -78,8 +78,7 @@ static void checkOidcProviderId(String providerId) {
     checkArgument(!Strings.isNullOrEmpty(providerId), "Provider ID must not be null or empty.");
     checkArgument(providerId.startsWith("oidc."),
         "Invalid OIDC provider ID (must be prefixed with 'oidc.'): " + providerId);
-    checkArgument(!providerId.equals("oidc.") && Utils.isSafeUrlSegment(providerId),
-        "Invalid OIDC provider ID: " + providerId);
+    checkArgument(Utils.isSafeUrlSegment(providerId), "Invalid OIDC provider ID: " + providerId);
   }
 
   static Map<String, Boolean> ensureResponseType(Map<String,Object> properties) {

src/main/java/com/google/firebase/auth/SamlProviderConfig.java

@@ -85,8 +85,7 @@ static void checkSamlProviderId(String providerId) {
     checkArgument(!Strings.isNullOrEmpty(providerId), "Provider ID must not be null or empty.");
     checkArgument(providerId.startsWith("saml."),
         "Invalid SAML provider ID (must be prefixed with 'saml.'): " + providerId);
-    checkArgument(!providerId.equals("saml.") && Utils.isSafeUrlSegment(providerId),
-        "Invalid SAML provider ID: " + providerId);
+    checkArgument(Utils.isSafeUrlSegment(providerId), "Invalid SAML provider ID: " + providerId);
   }
 
   private static List<Object> ensureNestedList(Map<String, Object> outerMap, String id) {

src/main/java/com/google/firebase/auth/multitenancy/TenantAwareFirebaseAuth.java

@@ -16,12 +16,9 @@
 
 package com.google.firebase.auth.multitenancy;
 
-import static com.google.common.base.Preconditions.checkArgument;
-
 import com.google.api.core.ApiAsyncFunction;
 import com.google.api.core.ApiFuture;
 import com.google.api.core.ApiFutures;
-import com.google.common.base.Strings;
 import com.google.common.util.concurrent.MoreExecutors;
 import com.google.firebase.FirebaseApp;
 import com.google.firebase.auth.AbstractFirebaseAuth;

src/main/java/com/google/firebase/auth/multitenancy/TenantManager.java

@@ -22,7 +22,6 @@
 import com.google.api.client.http.HttpResponseInterceptor;
 import com.google.api.core.ApiFuture;
 import com.google.common.annotations.VisibleForTesting;
-import com.google.common.base.Strings;
 import com.google.firebase.FirebaseApp;
 import com.google.firebase.auth.FirebaseAuth;
 import com.google.firebase.auth.FirebaseAuthException;