This PR implements a configurable URL base system to enable easy redirection of all Wealthsimple API calls for testing purposes. Previously, all API endpoints used hardcoded URLs throughout the codebase, making it impossible to redirect calls to mock servers during unit testing.

Changes Made

New URLConfiguration Singleton Class

• Created URLConfiguration singleton that manages the base URL for all API endpoints
• Default base URL: https://api.production.wealthsimple.com/v1/
• Internal access setBaseURL() method for testing configuration
• Helper methods for creating URL, URLComponents, and string representations

Updated All API Classes

• Token.swift: Replaced hardcoded URLs for /oauth/token and /oauth/token/info
• WealthsimpleAccount.swift: Replaced hardcoded URL for /accounts
• WealthsimplePosition.swift: Replaced hardcoded URL for /positions
• WealthsimpleTransaction.swift: Replaced hardcoded URL for /transactions

Comprehensive Test Coverage

• Added 9 comprehensive tests covering all URLConfiguration functionality
• Tests verify singleton pattern, URL construction, and configuration persistence
• All existing functionality remains unchanged (backward compatibility maintained)

Usage for Testing

// Redirect all API calls to mock server
URLConfiguration.shared.setBaseURL("https://mock.test.server/v1/")

// All subsequent API calls will use the mock server
// Token.getToken(), WealthsimpleAccount.getAccounts(), etc.

Benefits

• Easy Mock Testing: Single line change redirects all API calls to test servers
• Clean Architecture: Centralized URL management instead of scattered hardcoded URLs
• Backward Compatible: No changes to existing public API
• Type Safe: Proper Swift access control with internal-only configuration method

Fixes #250.

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