Add excel.sheet formatting contract for WranglesXL

[ebhills]

Closes #1135

Paired client implementation: https://github.com/wrangleworks/WranglesXL/pull/1199
Related client issue: https://github.com/wrangleworks/WranglesXL/issues/1198

Summary

• add a nested formatting.columns contract to excel.sheet
• support align, number_format, bold, and checkbox
• preserve the formatting payload for WranglesXL
• validate formatting keys and values at runtime
• keep differently formatted writes as ordered output segments

Behavior and compatibility

Recipes without formatting are unchanged. Matching batched writes still combine into one dataframe payload. A formatting change for the same sheet and cell starts a new payload segment so rows retain their original order and formatting intent.

This changes only excel.sheet; the file connector formatting behavior is unaffected.

Validation

• .venv\\Scripts\\python.exe -m pytest tests\\connectors\\test_excel.py: 11 passed
• git diff --check origin/main...HEAD: passed

Rollback

Revert commit 4ae4e107 to remove the new contract and restore the previous aggregation behavior.

[mborodii-prog]

@ebhills I did few changes:
In wrangles/connectors/excel.py:

Renamed number_format → num_format in _FORMATTING_OPTIONS, the validation branch, and its error message.
Renamed the number_format property → num_format in the connector's JSON schema block (_schema["write"]), with an updated description noting it matches the file connector's column_formats.num_format.
Added a doc comment above _FORMATTING_OPTIONS/_ALIGNMENTS explaining the syntax is intentionally aligned with _formatting.py, and updated the top-level formatting schema description to say the same.
In tests/connectors/test_excel.py:

Updated test_excel_sheet_write_preserves_column_formatting to use num_format instead of number_format.
Added test_excel_sheet_formatting_options_match_xlsxwriter_syntax, which:
asserts FORMATTING_OPTIONS is a subset of the real set* properties on xlsxwriter.format.Format (introspected via dir()), so a future option added to excel.sheet that isn't a real XlsxWriter property fails CI;
asserts _ALIGNMENTS - {"general"} is a subset of the values XlsxWriter's set_align() actually accepts (introspected via typing.get_args() on its Literal type annotation, since set_align() silently no-ops on unrecognized strings rather than raising).

[ebhills]

@mborodii-prog - one more formatting option that we need in this first go is text wrap. Can you please add it too?

[mborodii-prog]

@ebhills Added text_wrap as a new boolean formatting option, with tests passing

[mborodii-prog]

@ebhills this PR is tested in DEV, also opened aditional PR for XL to support num_format and text_wrap. To test it by own in DEv you can use image dev-1.20.0rc56 in execute-recipe-dev. As for me we're good to go agead with that PR:
recipe
'''

write:

• excel.sheet:
  name: Results
  formatting:
  columns:
  Price:
  num_format: '$#,##0.00'
  align: center
  bold: true
  Discount:
  num_format: '0%'
  align: right
  Rating:
  num_format: '0.0'
  align: center
  In_Stock:
  align: right
  bold: true
  checkbox: true
  Description:
  text_wrap: true
  align: center
  Date_Added:
  num_format: 'yyyy-mm-dd'

'''

sample data:

output: