Add Input Validation for Currency Conversion Endpoints

[Nathydre]

Files to modify : backend/src/routes/currency.ts , backend/src/services/currencyConversion.ts
Acceptance Criteria :

• Validate currency codes are supported
• Validate amount is positive number
• Add proper error messages for invalid inputs
• All tests pass Why Important : Improves data integrity and user experience

[nissinanlung]

can i work on this issue

[grantfox-oss]

🦊 GrantFox — @nissinanlung has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #301)
2. Your PR will be reviewed by the nathydre21 maintainers

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