Add comprehensive unit test suite with fixtures instead of mocks

[Copilot]

Created solid unit test suite covering core quoptuna functionality using real data and fixtures rather than mocks for true integration testing.

Changes

Test Infrastructure

• conftest.py: Shared fixtures for classification datasets, train/test splits, trained models, and temporary files
• 64 new tests across 4 test files (37→101 total)
• Coverage improved from 37% to 38.44%

Test Coverage by Module

• test_data_utils.py (10 tests): Data loading, preprocessing, label transformation, utility functions
• test_data_preparation_extended.py (20 tests): Dataset initialization, column handling, custom scalers, output formats
• test_optimizer.py (8 tests): Initialization, Optuna integration, study persistence, error handling
• test_xai_extended.py (26 tests): Configuration, metrics (F1/precision/recall/MCC/kappa), state serialization

Bug Fix

• Fixed langchain import compatibility (langchain → langchain_core)

Documentation

• TESTING.md: Test structure, coverage stats, running guide, best practices

Example Usage

# Fixtures provide real, pre-configured test data
def test_data_preparation(sample_dataframe_dataset):
    df = sample_dataframe_dataset
    X = df.drop(columns=["target"])
    y = df["target"]
    
    data_prep = DataPreparation(
        dataset={"x": X, "y": y},
        x_cols=list(X.columns),
        y_col="target"
    )
    
    assert data_prep.x_train is not None
    assert len(data_prep.x_train) == 75  # 75% train split

Key Metrics

• 100% pass rate (101/101 tests)
• 100% coverage: backend/models.py
• 94% coverage: backend/utils/data_utils/prepare.py
• 81% coverage: backend/utils/data_utils/data.py
• 74% coverage: backend/xai/xai.py

All tests use scikit-learn datasets and real model training—no mocks.

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.