Add price validation for market rate fetchers

[Obiajulu-gif]

This pull request introduces a new validatePrice utility to ensure that all price values used in market rate fetchers are positive numbers. The change improves data integrity by validating external and internal price data before use. The utility is applied throughout the GHS and KES rate fetchers, and comprehensive tests are added to verify its behavior and error handling.

Validation improvements:

• Added a new validatePrice function in validation.ts to ensure that price values are positive numbers, throwing an error otherwise.
• Updated both GHSRateFetcher and KESRateFetcher to use validatePrice when handling all incoming rate data, including fallback and placeholder values. [1] [2] [3] [4] [5] [6]

Testing enhancements:

• Added a new test file validation.test.ts to assert correct and incorrect usages of validatePrice.
• Expanded ghsFetcher.test.ts to cover scenarios where invalid price data is returned from external APIs, ensuring errors are thrown as expected.

Closes #64

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[Sadeequ]

Please resolve all conflicts and also make sure your Types follows the repo standard for the CI check .

[Obiajulu-gif]

@Sadeequ i have fix it