test: Add unit tests for db folder

[Aravinda-HWK]

📌 Description

This PR is to add test cases for db folder. Test coverage: 79.6%

• Closes [TASK] Add Unit Tests for Database Layer #120

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🔍 Changes Made

• Add the test cases for the Database folder
• Update the ci.yaml file(more organized way)
• Update the Makefile

✅ Checklist (Email System)

• Core IMAP commands tested (LOGIN, CAPABILITY, LIST, SELECT, FETCH, LOGOUT).
• Authentication is tested.
• Docker build & run validated.
• Configuration loading verified for default and custom paths.
• Persistent storage with Docker volume verified.
• Error handling and logging verified
• Documentation updated (README, config samples).

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🧪 Testing Instructions

To test the server, use the instructions in the README in the test directory.

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📷 Screenshots / Logs (if applicable)

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⚠️ Notes for Reviewers