chore(pkg-py): remove stale ggsql casing guidance

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Context

This is a follow-up to the earlier Python-side ggsql 0.3 prep work in #225.

That upgrade moved QueryChat onto the newer ggsql validation and syntax behavior, but it left behind one stale workaround from the pre-0.3 era: guidance and execution logic that assumed ggsql column references in VISUALISE were effectively case-sensitive and therefore needed QueryChat to normalize or teach lowercase aliases.

In other words, this is effectively a follow-up to a follow-up:

• chore(pkg-py): prepare querychat for ggsql 0.3.0 #225 updated QueryChat for ggsql 0.3 semantics
• this PR finishes the casing-related cleanup that should have come along with that upgrade

Current ggsql now handles uppercase column references correctly, so QueryChat no longer needs to:

• lowercase dataframe columns before registering them with ggsql
• teach the model a special ROOM_TYPE/lowercase-alias workaround
• repeat the redundant charts-vs-tables note in the ggsql syntax reference

Summary

• remove the stale column-casing workaround from Python ggsql execution
• remove obsolete casing/charts-vs-tables guidance from the Python ggsql syntax prompt
• add regression tests for uppercase column support and prompt cleanup

Testing

• uv run pytest -q pkg-py/tests/test_viz_tools.py pkg-py/tests/test_ggsql.py -p no:playwright
• uv run ruff check pkg-py/src/querychat/_viz_ggsql.py pkg-py/tests/test_ggsql.py pkg-py/tests/test_viz_tools.py --config pyproject.toml