feat: Add PEP 561 type stubs (.pyi files) for better IDE autocompletion

## Summary
The SDK does not ship a PEP 561 marker (`py.typed`) or stub files (`.pyi`), which means IDEs like VS Code and PyCharm cannot provide accurate autocompletion, type hints, or inline docs for SDK users.

## Current Behavior
Users installing via `pip install hiero-sdk-python` get no type information in their IDE. `mypy` is used internally but the package does not advertise itself as typed.

## Expected Behavior
- Add a `py.typed` marker file to `src/hiero_sdk_python/`
- Ensure all public API functions have complete type annotations
- Verify `mypy --strict` passes on the public API surface

## Impact
- Affects every downstream developer using the SDK
- Low effort, high value — PEP 561 compliance is a one-file change plus annotation cleanup

## References
- PEP 561: https://peps.python.org/pep-0561/
- mypy docs on distributing types: https://mypy.readthedocs.io/en/stable/installed_packages.html

## Environment
- SDK version: v0.2.6