SSO Refactoring (#2818)

# Description of Changes

* Refactoring of SSO code around OAuth & SAML 2
* Enabling auto-login with SAML 2 via the new `SSOAutoLogin` property
* Correcting typos & general cleanup

---

## Checklist

### General

- [x] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [x] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md)
(if applicable)
- [x] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/HowToAddNewLanguage.md)
(if applicable)
- [x] I have performed a self-review of my own code
- [x] My changes generate no new warnings

### Documentation

- [x] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [x] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [x] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md#6-testing)
for more details.

src/test/java/stirling/software/SPDF/utils/validation/ValidatorTest.java

@@ -0,0 +1,54 @@
+package stirling.software.SPDF.utils.validation;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+import org.mockito.junit.jupiter.MockitoExtension;
+import stirling.software.SPDF.model.UsernameAttribute;
+import stirling.software.SPDF.model.provider.GitHubProvider;
+import stirling.software.SPDF.model.provider.GoogleProvider;
+import stirling.software.SPDF.model.provider.KeycloakProvider;
+import stirling.software.SPDF.model.provider.Provider;
+
+import java.util.List;
+import java.util.stream.Stream;
+
+import static org.junit.jupiter.api.Assertions.*;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+@ExtendWith(MockitoExtension.class)
+class ValidatorTest {
+
+    @Test
+    void testSuccessfulValidation() {
+        var provider = mock(GitHubProvider.class);
+
+        when(provider.getClientId()).thenReturn("clientId");
+        when(provider.getClientSecret()).thenReturn("clientSecret");
+        when(provider.getScopes()).thenReturn(List.of("read:user"));
+
+        assertTrue(Validator.validateProvider(provider));
+    }
+
+    @ParameterizedTest
+    @MethodSource("providerParams")
+    void testUnsuccessfulValidation(Provider provider) {
+        assertFalse(Validator.validateProvider(provider));
+    }
+
+    public static Stream<Arguments> providerParams() {
+        Provider generic = null;
+        var google = new GoogleProvider(null, "clientSecret", List.of("scope"), UsernameAttribute.EMAIL);
+        var github = new GitHubProvider("clientId", "", List.of("scope"), UsernameAttribute.LOGIN);
+
+        return Stream.of(
+                Arguments.of(generic),
+                Arguments.of(google),
+                Arguments.of(github)
+        );
+    }
+
+}