fix(excel): restore split-by-column fallback

[lawliet206]

Problem

office.excel.split_excel_by_column() and the matching Excel Skill fail before processing any workbook when used with the currently published poexcel 0.0.22 package:

AttributeError: module 'poexcel' has no attribute 'split_excel_by_column'

The repository already contains a maintained split-by-column implementation under office.lib.excel.SplitExcel, but the public wrapper does not use it.

Root Cause

The wrapper unconditionally calls poexcel.split_excel_by_column. The current poexcel release does not expose that name from its top-level package, so attribute lookup fails before the bundled implementation can run.

Solution

• Continue using poexcel.split_excel_by_column when a future or alternate poexcel version exposes it.
• Fall back lazily to the bundled office.lib.excel.SplitExcel.split_excel_by_column implementation when the dependency does not expose the function.
• Add regression tests for both delegation paths.

Testing

python -m unittest tests.test_code.test_excel_split_wrapper -v
2 tests passed

python -m unittest tests.test_code.test_instruction_url tests.test_code.test_web tests.test_code.test_excel_split_wrapper -v
5 tests passed

python -m compileall -q office tests/test_code/test_excel_split_wrapper.py
passed

Also verified the fallback with poexcel 0.0.22 and a generated .xlsx workbook. It created the split output with the expected Alpha and Beta worksheets, and the input/output files could be renamed afterward (no open file handles remained).

Compatibility

No public API or parameter behavior changes. Environments where poexcel already exports the function keep the existing delegation path; affected environments regain the documented functionality through the bundled implementation.

Related Issue

No dedicated issue. This is a follow-up to the bundled split implementation fixes merged in #161.