Add Excel/document preprocessing coverage and UI error-contract tests

[rad1092]

Motivation

• Validate end-to-end handling of CSV/Excel/document inputs (single/multi-sheet, empty sheets, header validation) and ensure document extraction fallback and API/UI error contract are consistent.
• Make supported input scope, constraints and recommended file preparation explicit in the README for users uploading CSV/Excel/document files.

Description

• Add comprehensive web-layer tests in tests/test_web.py including an XLSX generator, a local HTTP server helper, and tests for Excel single-sheet parsing, multi-sheet selection, empty-sheet validation, header empty/duplicate checks, document extract success/failure payloads, and /api/analyze fallback error contract handling.
• Add tests/test_ui_contract.py to statically verify UI-side error detail priority (error_detail first) and presence of key failure status messages used by the frontend.
• Update README.md with an "업로드 입력 지원 범위 / 제약 / 준비 권장사항" section describing supported formats, limitations (e.g. only .xlsx), error behaviors, and recommended file preparation steps.

Testing

• Ran pytest -q tests/test_web.py tests/test_document_extract.py tests/test_ui_contract.py and all tests passed: 16 passed in 1.21s.
• Changes are tests and documentation only; no production runtime behavior was modified by this PR.

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Codex Task