🧪 add unit tests for sha256 function

[lsb11]

🎯 What: The testing gap addressed
The sha256 utility function in functions/api/submit-gap.js lacked unit tests.

📊 Coverage: What scenarios are now tested

• Known IP string ("127.0.0.1")
• Empty string edge case ("")
• String containing non-ASCII/unicode characters ("hello world 🚀") to test TextEncoder handling.
• Determinism check (ensures calling sha256 twice with the same input yields the same result).

✨ Result: The improvement in test coverage
The pure string hashing function is now completely tested and verified for stability and correctness. We also added vitest to devDependencies and set up the "test" script in package.json.

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PR created automatically by Jules for task 10078211780660741736 started by @lsb11

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