Restore default Excel table formatting in file connector

[Copilot]

Linked issue

What changes

Excel files written without explicit formatting no longer received the file connector’s default table styling.

• Restores Table Style Medium9 and top alignment using XlsxWriter.
• Preserves pandas Excel options, mixed-type columns, indexes, and in-memory output.
• Handles duplicate, nullable, and MultiIndex headers safely.
• Keeps explicit advanced formatting on the existing Polars/XlsxWriter path.

write:
  - file:
      name: output.xlsx

This now produces a formatted Excel table without requiring a formatting block.

How it was verified

• 74 focused file and Excel connector tests.
• Full credential-safe suite: 1,481 passed, 3 skipped.
• Manual workbook inspection of table style, range, alignment, sheet name, and values.
• Secret scan and CodeQL completed with no findings.

Compatibility and risk

• No migration required.
• Explicit formatting behavior remains unchanged.
• constant_memory=True now raises ValueError because it is incompatible with pandas column writes and Excel tables.
• MultiIndex worksheets remain valid but omit the incompatible table overlay.
• Safest rollback: revert the formatting commits.

Ready-for-review checklist

• One human delivery owner is assigned
• The linked issue and intended milestone are correct
• The branch is current with main and has no merge conflicts
• Focused tests pass
• New or changed behavior has direct test coverage
• Documentation/schema/configuration is updated where applicable
• The PR contains no unrelated changes
• The PR description reflects the branch's current scope and latest validation
• One primary reviewer is requested only when this PR is ready

See the pull request workflow.

• Fixes Table format in File even when formatting not specified #1139