🧪 [testing] Add tests for submit-gap endpoint

[lsb11]

🎯 What: Added a new test file functions/api/submit-gap.test.js to cover the submit-gap endpoint using Vitest.
📊 Coverage: The test suite covers happy paths, CORS headers, input validation errors (e.g. invalid json, out of bounds numeric checks), database execution with mocked bindings (vi.fn()), and database failure scenarios.
✨ Result: Improved test coverage and reliability of the Cloudflare Pages Function endpoint.

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PR created automatically by Jules for task 10817903700200840626 started by @lsb11

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[cloudflare-workers-and-pages]

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