examples: add current time tool example - #798

What

Adds examples/tools-current-time.py, an example showing how to give a model a
get_current_time tool, plus a unit test covering tool schema generation for a
function with a defaulted str argument.

Why

Models have no clock, so questions like "what time is it?" get guessed or wrong
answers. Tool calling is the correct fix, and there is currently no example
showing this common use case.

How I tested

• Ran the example against llama3.1 locally; the model called the tool with a
  timezone and answered correctly from the tool output.
• Added a test in tests/test_utils.py; pytest and ruff pass.