fix: defer MongoClient init to request time to fix build crash

[Harshit-Mishra2212]

📌 Description

new MongoClient(uri) was called at module level in cohort/route.ts with a non-null assertion on process.env.MONGO_URI. During next build, Next.js evaluates route modules without runtime environment variables, so MONGO_URI is undefined at build time and MongoClient throws immediately crashing the entire build:

TypeError: Cannot read properties of undefined (reading 'startsWith')
at new MongoClient (mongodb/lib/mongo_client.js:52:61)
at app/api/cohort/route.js
Build error occurred: Failed to collect page data for /api/cohort

Reproducible on any clean clone without .env.local - exactly the environment CI pipelines and Vercel production builds run in. The ! non-null assertion only silences TypeScript, it does nothing at runtime.

Additionally, client.connect() was called on every GET request with no connection reuse, opening a new MongoDB connection per request and never closing it exhausting the Atlas connection pool under real load.

Not related to any specific issue but directly affects deployment reliability.

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✅ Changes

• Fix

• Moved MongoClient creation into a getClient() helper that runs only at request time - build no longer crashes when MONGO_URI is absent

• Added explicit MONGO_URI presence check that returns a meaningful error message instead of crashing with a TypeError

• Added module-level cachedClient so the connected client is reused across requests - standard recommended pattern for MongoDB in Next.js serverless deployments

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🔍 Additional Notes

No breaking changes. The GET handler behaviour is identical only the initialisation timing and connection reuse strategy changed. Existing callers of /api/cohort are unaffected.

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Checklist

• I have tested this code
• I have added necessary documentation
• I have linked relevant issue(s)
• I followed the code style of the project
• I reviewed my own code

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