Add unit test for account service

[NeatGuyCoding]

Summary

This PR adds comprehensive unit tests for the account service module, significantly improving test coverage for critical account-related operations. The implementation includes three main test classes covering AccountService, TenantService, and RegisterService with extensive test scenarios for authentication, account management, tenant operations, and user registration workflows.

Key Features Added:

• Complete test coverage for account authentication, creation, and password management
• Tenant management tests including creation, member management, and permission checks
• Registration service tests covering system setup, user registration, and member invitation flows
• Mock factory pattern for consistent test data creation
• Comprehensive error handling tests for various edge cases and failure scenarios
• Database operation verification with proper transaction handling
• External service integration mocking for feature services, billing, and passport services

Test Categories Covered:

1. Authentication Tests: Login validation, password verification, account status checks
2. Account Creation Tests: User registration with various parameters and validation
3. Password Management Tests: Password updates with security validations
4. User Loading Tests: Account retrieval with tenant associations
5. Tenant Management Tests: Workspace creation, member management, role updates
6. Registration Flow Tests: Complete user onboarding including OAuth integration
7. Invitation System Tests: Member invitation workflow with token management

Technical Implementation:

• Uses pytest fixtures for consistent mock setup across test methods
• Implements a factory pattern (TestAccountAssociatedDataFactory) for creating test data
• Provides helper methods for database operation verification
• Includes comprehensive error scenario testing with proper exception handling
• Follows best practices for unit testing with proper isolation and mocking

This enhancement ensures robust testing coverage for the account service layer, improving code reliability and maintainability while providing clear examples of expected behavior for future development.

Screenshots

Before	After
No unit tests for account services	Comprehensive test suite with 50+ test methods covering all major account operations

Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods