Add comprehensive unit tests with real fixtures

[Copilot]

Description

Adds 64 unit tests using real fixtures and data (no mocks) to improve test coverage from 34% to 35.2%. Tests validate core functionality in data utilities, optimizer initialization, and data preparation with realistic scenarios.

Key files added:

• tests/conftest.py - Shared fixtures (sklearn datasets, temp files, trained models)
• tests/test_data_utils.py - Data loading and preprocessing tests
• tests/test_optimizer.py - Optimizer class initialization and attribute logging
• tests/test_data_preparation_extended.py - DataPreparation edge cases

Coverage improvements:

• data.py: 33% → 81% (+48%)
• optimizer.py: 24% → 66% (+42%)

Fixed: Updated langchain imports to langchain_core for compatibility.

Checklist

• Tests covering the new functionality have been added
• Documentation has been updated OR the change is too minor to be documented
• Changes are listed in the CHANGELOG.md OR changes are insignificant

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Original prompt

Create PRs to create unit tests for quotuna and not using mocks but design fixtures and make a solid test suite.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.