Add comprehensive unit tests for ExchangeRateProvider with MCP Server integration

[rstropek]

Description

The ExchangeRateProvider in the Xchg project needs comprehensive unit tests to ensure reliability and correctness when integrating with the Exchange Rate Provider MCP Server.

Scope

• Add unit tests for all public methods of ExchangeRateProvider
• Test error handling scenarios (invalid rates, missing currencies, etc.)
• Test integration with MCP Exchange Rate Server data
• Ensure proper validation of exchange rates and currency codes
• Test currency management (add, remove, update)

Implementation Details

• Tests should be added to the xchg-logic crate
• Use realistic exchange rate data from the MCP server for testing
• Cover edge cases like invalid rates, duplicate currencies, and missing data
• Ensure proper error handling and meaningful error messages

Acceptance Criteria

• All public methods of ExchangeRateProvider are tested
• Error scenarios are properly covered
• Tests use real exchange rate data from MCP server
• All tests pass successfully
• Code coverage is adequate for the module

Files to modify

• xchg-logic/src/lib.rs (if test module needs to be added)
• Add test files in xchg-logic/src/ or xchg-logic/tests/