Fix mypy type checking issues

[amithad]

Running mypy on the project reveals type checking issues that need to be addressed to ensure type safety and code quality.

1. Run:

cd ak-py && uv run mypy src/agentkernel

2. Fix the listed issues
3. Repeat the same for example codes
4. Add mypy check to makeFile's lint and lint check commands (They'll run automatically when raising PRs).