Upgrade to Python 3.10+ (3.9 broken anyway)

agent-evaluation currently specifies Python 3.9+, as per:
- [setup.py install_requires](https://github.com/awslabs/agent-evaluation/blob/5d2784af28bae4bc3a0edd651ce78def310faea1/setup.py#L13)
- [setup.py classifiers](https://github.com/awslabs/agent-evaluation/blob/5d2784af28bae4bc3a0edd651ce78def310faea1/setup.py#L50)
- [github CI](https://github.com/awslabs/agent-evaluation/blob/5d2784af28bae4bc3a0edd651ce78def310faea1/.github/workflows/ci.yml#L8)
- [installation docs](https://github.com/awslabs/agent-evaluation/blob/5d2784af28bae4bc3a0edd651ce78def310faea1/docs/installation.md)
- [sample streamlit app readme](https://github.com/awslabs/agent-evaluation/blob/5d2784af28bae4bc3a0edd651ce78def310faea1/samples/streamlit_app/README.md)

...but:
1. Python 3.9 will [stop receiving security updates in 2025-10](https://devguide.python.org/versions/), and
2. Support is already broken by various uses of `|` in type expressions - which was only introduced in [Python 3.10 in PEP604](https://peps.python.org/pep-0604/). When I try to run pytest on a 3.9 environment, I see a slew of `TypeError: unsupported operand type(s) for |: 'type' and 'NoneType'` errors.

...So I probably suggest to just bump the listed dependency to 3.10+?

Review the Python version declarations in setup.py, .github/workflows/ci.yml, docs/installation.md, and samples/streamlit_app/README.md. Run pytest in a Python 3.9 environment to confirm the reported failures, then verify that all listed metadata, CI configuration, and documentation consistently require Python 3.10+ and that the test suite passes on the supported version.

Written by the indexing model from the issue text.