Add backend test suite and pytest configuration

[pragati-0208]

This PR introduces a structured backend test suite and improves project stability.

Changes include:

• Added modular pytest test suite (chat, health, general tests)
• Implemented async mocking for chat endpoint
• Configured pytest in pyproject.toml
• Added backend package initialization (init.py)
• Fixed datetime deprecation warnings

All tests pass locally (9/9).

[QuantumByte-01]

Closing as a duplicate of #64 which has a more complete test suite. This PR also changes import paths to backend.agents package-style in main.py which breaks the existing run setup, and the test file has commented-out code blocks left in. Please contribute improvements on top of #64 instead.