Add /optimize FastAPI endpoint with tests

[suyash469]

Summary

This PR adds a minimal FastAPI /optimize endpoint that runs a Least Squares index-replication optimization on MSCI data and returns optimized weights.

• Adds /optimize in src/api.py:

  • Uses load_data_msci to load MSCI country and benchmark returns.
  • Sets LongOnly, fully-invested constraints with Constraints.
  • Builds OptimizationData(return_series, bm_series, align=True).
  • Runs a LeastSquares optimization and returns assets, weights, and sum_weights.

• Adds tests:

  • test/tests_api.py:
    • Tests / and /optimize using TestClient, mocking load_data_msci and LeastSquares.
  • test/tests_data_loader.py:
    • Uses unittest.mock.patch on pd.read_csv, so the test no longer depends on local CSV files.

Testing

• python -m unittest test.tests_data_loader test.tests_api

Related to: #26