Fix: Improve Reliability of Currency Exchange Rate Fetching

[TanCodeX]

Summary

Closes #226

This PR improves the robustness of the currency exchange rate fetching logic in src/context/AppContext.jsx.

Previously, the app fetched data from the external API without validating the HTTP response or ensuring the returned payload was valid. This could lead to silent failures or incorrect exchange rate data being used in the application.

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Changes Made

• Added explicit check for res.ok before processing response
• Validated API response structure to ensure returned JSON is correct
• Added handling for known API error payloads (e.g., rate limit or invalid response formats)
• Introduced descriptive error throwing when validation fails
• Wrapped fetch logic in proper error handling to catch and manage failures gracefully

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Why This Matters

• Prevents silent failures in currency conversion logic

• Ensures only valid exchange rate data is used in the app

• Improves resilience against:

  • API downtime
  • Rate limiting
  • Malformed responses

• Makes debugging easier with clear error messages

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Behavior Change

• Before: Invalid API response → silent failure or incorrect data ❌
• Now: Invalid API response → caught and handled gracefully with error logging ✅

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Testing

• Simulate API downtime → app should not crash
• Simulate rate limit response → error should be caught and handled
• Valid response → exchange rates load correctly
• Invalid JSON structure → fallback/error handling triggered

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Impact

• Improved data reliability for currency conversion
• Better error visibility for debugging
• Enhanced stability for financial features

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[TanCodeX]

@khanirfan18 Please review!!

[TanCodeX]

@khanirfan18 review!