Python: Fix Handlebars messages helper ignoring its argument - #14537

Motivation and Context

The Handlebars messages helper ignores its argument. {{messages my_history}} always reads chat_history from the context, so it raises KeyError when that variable is missing and renders the wrong history when both exist. The Jinja2 messages(history) helper uses its argument.

Fixes #14535

Description

_messages in handlebars_system_helpers.py now uses the first positional argument. If no argument is given it falls back to the chat_history context variable, so existing {{messages chat_history}} templates behave the same. The unused options parameter was removed. For a non-block helper pybars passes the first argument in that slot, which is why the argument was never visible. A bare {{messages}} used to raise TypeError and now works through the fallback.

Added two tests (argument with a different variable name, and argument preferred over a chat_history variable). Both fail on main. tests/unit/prompt_template, tests/unit/kernel and tests/unit/functions pass. ruff check clean on the touched files, mypy clean on the helper module.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄