docs: Add changelog for composite primary key support in dbt export

[toshifumisuga]

Summary

This PR adds the changelog that was missing in the previous one.
#898

• Tests pass
• ruff format

Tests and ruff result

(datacontract-cli)datacontract-cli toshifumi.suga$ pytest tests/test_export_dbt_models.py -v
================================================================================================================= test session starts =================================================================================================================
platform darwin -- Python 3.11.13, pytest-8.4.2, pluggy-1.6.0 -- /Users/toshifumi.suga/Documents/dev/datacontract-cli/.venv/bin/python3
cachedir: .pytest_cache
rootdir: /Users/toshifumi.suga/Documents/dev/datacontract-cli
configfile: pyproject.toml
plugins: xdist-3.8.0, anyio-4.10.0, typeguard-4.4.4
collected 6 items                                                                                                                                                                                                                                     

tests/test_export_dbt_models.py::test_cli PASSED                                                                                                                                                                                                [ 16%]
tests/test_export_dbt_models.py::test_to_dbt_models PASSED                                                                                                                                                                                      [ 33%]
tests/test_export_dbt_models.py::test_to_dbt_models_with_server PASSED                                                                                                                                                                          [ 50%]
tests/test_export_dbt_models.py::test_to_dbt_models_with_no_model_type PASSED                                                                                                                                                                   [ 66%]
tests/test_export_dbt_models.py::test_to_dbt_models_with_model_level_composite_primary_key PASSED                                                                                                                                               [ 83%]
tests/test_export_dbt_models.py::test_to_dbt_models_with_single_column_primary_key PASSED                                                                                                                                                       [100%]

================================================================================================================== 6 passed in 1.79s ==================================================================================================================
(datacontract-cli)datacontract-cli toshifumi.suga$ ruff check tests/test_export_dbt_models.py 
All checks passed!
(datacontract-cli)datacontract-cli toshifumi.suga$ 

• README.md updated (if relevant)
•  CHANGELOG.md entry added

[toshifumisuga]

The CI is failing with errors that I believe are unrelated to my changes. I'd appreciate it if you could verify.
Let me know if any action is required from me

[toshifumisuga]

The sample was not using generic table and column names, so I corrected them together.

[jochenchrist]

The CI is failing with errors that I believe are unrelated to my changes. I'd appreciate it if you could verify. Let me know if any action is required from me

Yes, correct. Sorry for inconvienience, I will fix main.