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

- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

Author: VANDRANKI

python/semantic_kernel/prompt_template/utils/handlebars_system_helpers.py

@@ -10,12 +10,15 @@
 logger: logging.Logger = logging.getLogger(__name__)
 
 
-def _messages(this, options, *args, **kwargs):
+def _messages(this, *args, **kwargs):
     from semantic_kernel.contents.chat_history import ChatHistory
 
-    if not isinstance(this.context["chat_history"], ChatHistory):
+    # Use the argument passed to the helper, e.g. {{messages my_history}}.
+    # Fall back to the "chat_history" variable when no argument is given.
+    chat_history = args[0] if args else this.context.get("chat_history")
+    if not isinstance(chat_history, ChatHistory):
         return ""
-    return this.context["chat_history"].to_prompt()
+    return chat_history.to_prompt()
 
 
 def _message_to_prompt(this, *args, **kwargs):

python/tests/unit/prompt_template/test_handlebars_prompt_template.py

@@ -479,3 +479,37 @@ async def test_safe_types_are_allowed():
 
     assert "42" in result
     assert "true" in result
+
+
+async def test_helpers_chat_history_messages_uses_argument(kernel: Kernel):
+    template = """{{messages history}}"""
+    target = create_handlebars_prompt_template(template, allow_dangerously_set_content=True)
+    chat_history = ChatHistory()
+    chat_history.add_user_message("User message")
+    rendered = await target.render(kernel, KernelArguments(history=chat_history))
+    assert (
+        rendered.strip() == """<chat_history><message role="user"><text>User message</text></message></chat_history>"""
+    )
+
+
+async def test_helpers_chat_history_messages_prefers_argument_over_chat_history_variable(kernel: Kernel):
+    template = """{{messages other}}"""
+    target = create_handlebars_prompt_template(template, allow_dangerously_set_content=True)
+    wrong = ChatHistory()
+    wrong.add_user_message("WRONG")
+    right = ChatHistory()
+    right.add_user_message("RIGHT")
+    rendered = await target.render(kernel, KernelArguments(chat_history=wrong, other=right))
+    assert "RIGHT" in rendered
+    assert "WRONG" not in rendered
+
+
+async def test_helpers_chat_history_messages_without_argument_uses_chat_history_variable(kernel: Kernel):
+    template = """{{messages}}"""
+    target = create_handlebars_prompt_template(template, allow_dangerously_set_content=True)
+    chat_history = ChatHistory()
+    chat_history.add_user_message("User message")
+    rendered = await target.render(kernel, KernelArguments(chat_history=chat_history))
+    assert (
+        rendered.strip() == """<chat_history><message role="user"><text>User message</text></message></chat_history>"""
+    )