Preserve Excel export and append row placement

[rudycelekli]

Description

Excel export honors start_row for new files and appends after the worksheet's actual final row. Appending respects formatted template rows, and new exports start at the requested worksheet offset.

How has this been tested?

• Added native regression tests that fail against develop ae0268771f76 and pass with this change.
• Ran the regression and adjacent repository tests: 134 passed in 4.52s.
• Ruff lint/format and git diff --check pass.
• Tests use repository source with declared dependencies and isolated temporary settings; no live provider credentials or data services.

Full provider integrations were not run.

Additional compatibility checks: the combined production changes pass the full core suite (2,270 tests, 36 skipped, 184 subtests). The canonical CLI suite passes 2,241 tests with one existing backend test failure reproduced on unchanged develop. Individual native regression and adjacent tests pass; changed production files and new regression tests pass Ruff and ty. Existing OpenAPI test typing diagnostics also reproduce on unchanged develop. No live data-provider integrations were exercised.

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