fix: Add packaging to base dependencies (#897)

# Description
This PR resolves an issue where installing the `a2a-sdk` from a fresh
environment resulted in a ModuleNotFoundError: No module named
'packaging' error at runtime.

The packaging library is a core runtime requirement; it is imported by
several components such as `client_factory.py`, `versions.py`, and
`helpers.py` to parse and validate protocol versions (e.g.,
Version(VERSION_STRING)). However, it was previously omitted from the
base dependencies in `pyproject.toml`.

This has been resolved by adding `packaging>=24.0` to the base
dependencies list in `pyproject.toml`.

Packaging dependency was first introduced here:
https://github.com/a2aproject/a2a-python/blob/e4b365306d554014bbb2195ad2a3d3c85e01cf78/src/a2a/client/client_factory.py#L10

# Changes Made
Added "packaging>=24.0" to the dependencies array in pyproject.toml.

Author: sokoliva

pyproject.toml

@@ -16,6 +16,7 @@ dependencies = [
   "json-rpc>=1.15.0",
   "googleapis-common-protos>=1.70.0",
   "culsans>=0.11.0 ; python_full_version < '3.13'",
+  "packaging>=24.0",
 ]
 
 classifiers = [