This PR adds comprehensive test coverage for the WealthsimpleAccount struct, following the established pattern from TokenTests. The implementation includes 18 new test cases that thoroughly exercise all public methods and error handling paths.

What's Added

New Test File: WealthsimpleAccountTests.swift

• 18 comprehensive test cases covering all aspects of WealthsimpleAccount.getAccounts()
• Helper methods to reduce code duplication and improve maintainability
• Extended MockURLProtocol to support the /accounts endpoint

Test Coverage

Successful Scenarios (2 tests)

• Valid JSON responses with proper account data validation
• Multiple account types: ca_tfsa, ca_rrsp, ca_cash_msb, ca_cash, ca_non_registered

Network & HTTP Error Handling (4 tests)

• Network connection failures (URLError.networkConnectionLost)
• HTTP error status codes (401, 500, etc.)
• Invalid response types (non-HTTPURLResponse)
• Empty data responses

JSON Parsing Error Coverage (5 tests)

• Invalid JSON syntax
• Wrong JSON structure (arrays instead of objects)
• Missing required top-level fields (results, object)
• Invalid field values (wrong object type)

Individual Account Validation (6 tests)

• Missing required account fields (id, type, base_currency, custodian_account_number)
• Invalid account type values
• Invalid account object values

Edge Cases (1 test)

• Empty results array handling

Technical Implementation

The tests follow the same patterns established in TokenTests:

• Uses MockURLProtocol for network request interception
• Creates valid tokens through MockCredentialStorage
• Uses XCTestExpectation for async testing
• Validates all AccountError enum cases
• Tests property access and data mapping

Error Coverage

All AccountError cases are tested:

• noDataReceived
• httpError(error: String)
• invalidJson(error: String)
• invalidJsonType(json: Any)
• missingResultParamenter(json: [String: Any])
• invalidResultParamenter(json: [String: Any])
• tokenError(_ error: TokenError)

Test Results

• ✅ All 18 new tests pass
• ✅ All existing tests continue to pass (42 total tests)
• ✅ Code builds successfully with strict warnings
• ✅ Follows Swift coding conventions and project style

The implementation provides thorough validation of the WealthsimpleAccount struct's behavior under both normal and error conditions, ensuring robust error handling and proper data parsing.

Fixes #257.

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