fix(auth): dynamically resolve field-specific conflicts from MongoDB E11000 duplicate keys

[Aryan0819]

Related Issue

• Closes: 🐛 Bug Report: Unhandled Validation Errors / Server Crash on Duplicate Email #677

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Description

🧱 The Architectural Context:
When schemas enforce unique data parameters (like unique: true on an email property), MongoDB creates an indexed constraint network to preserve data integrity across collections.

❌ The Failure Mechanism:
If a new user attempts to sign up with an email address or username that already exists, the database rejects the insertion block and throws a raw internal network error: MongoServerError: E11000 duplicate key error. Because our controller logic lacked an error filter to isolate this specific code, Express would halt route execution immediately upon rejection.

💥 The Impact:
The application layer defaults to an unformatted 500 Internal Server Error payload. This breaks user navigation on the frontend UI with a generic error indicator and leaks raw, sensitive system database architectures into production application log streams.

✅ The Solution:
Implemented an indexed error pattern interceptor directly inside the authentication storage block's catch parameters. The filter captures error code 11000, overrides the default server failure sequence, and dynamically extracts the conflicting property using Object.keys(err.keyValue)[0] to send a precise, customer-facing 400 Bad Request validation message back down the client network.

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How Has This Been Tested?

Duplicate Registration Rejection: Attempted creating a second profile using an email address already registered in the system. Verified that the API instantly rejects the submission with a 400 status and returns the clear message string.

Fresh Account Integrity: Confirmed that signing up with a unique, unregistered email address continues to bypass the error block and commits cleanly to MongoDB.

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Type of Change

• Bug fix
• New feature
• Code style update
• Breaking change
• Documentation update

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 2f939dab-db05-4cc6-87fc-bfcfd26631b4

📥 Commits

Reviewing files that changed from the base of the PR and between 53f820b and 3a8039f.

📒 Files selected for processing (2)

• backend/routes/auth.js
• backend/server.js

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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