Fix ABLE feedback UserMessage import

[DavidJBianco]

Motivation

• The Streamlit ABLE feedback path imported UserMessage from autogen_agentchat.messages, which can raise an ImportError (and crash the /api/able-table flow) because the ABLE assistant and model client expect UserMessage from autogen_core.models.

Description

• Replace the UserMessage import in peak_assistant/streamlit/util/helpers.py to from autogen_core.models import UserMessage while keeping TextMessage from autogen_agentchat.messages to preserve existing behavior.
• Update tests/unit_tests/test_chat_history_conversion.py to import and assert against autogen_core.models.UserMessage so tests reflect the corrected message type.

Testing

• Ran uv run --python 3.12 pytest tests/unit_tests/test_chat_history_conversion.py which passed (10 passed).
• Ran uv run --python 3.12 ruff check tests/unit_tests/test_chat_history_conversion.py and linter issues were resolved before committing and checks passed.
• Noted environment-specific issues when running pytest with the ambient Python 3.14 (missing autogen_agentchat dependency) and a failed pyarrow build when trying to run in the ambient 3.14 environment; these are dependency/build environment problems and not caused by the change.

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Codex Task