fix(knowledge): tolerate invalid XLSX creation metadata

[chqcjq]

Summary

Fixes #1878.

Some XLSX exporters put a non-date value such as ASUS in docProps/core.xml's dcterms:created. The workbook's cells can be perfectly readable, but openpyxl rejects the entire document while reading properties, before knowledge extraction can reach the worksheets.

Use a per-call ExcelReader subclass that validates this one field with openpyxl's own DocumentProperties date validator. If it raises ValueError, remove only that creation element from the in-memory properties XML and log a warning. All remaining metadata, styles and worksheet parsing still use openpyxl's normal path.

• Source bytes are never changed, and no process-wide monkeypatch is used.
• Normal read_only=True / data_only=True options remain unchanged.
• Close the input archive on both reader/extraction success and failure.
• Other metadata failures and damaged worksheets remain errors.
• Includes a single Unreleased CHANGELOG entry.

The issue is filed as a compatibility request; this implements its selective creation-metadata tolerance, rather than treating arbitrary malformed workbooks as valid.

Issue / existing PR checks

• [feature] 知识库导入 XLSX 时对无关的创建时间元数据异常提供受限容错 #1878 remains open; no associated PR in its timeline when checked.
• Searched open PR descriptions and all-state PR search for [feature] 知识库导入 XLSX 时对无关的创建时间元数据异常提供受限容错 #1878 / XLSX creation metadata / read_properties; no existing fix found.
• Base: develop@99e479a6b45ecf6f13a1abf32db4f2b5787240e8.

Verification

Windows / Python 3.12 / frozen uv.lock (openpyxl 3.1.5):

• Before fix: both XLSX and XLSM synthetic workbook tests fail with could not read properties after changing only the creation timestamp to ASUS. The damaged-sheet control also fails at properties before reaching the intended worksheet error.

• After fix: uv run --no-sync python -m pytest tests/unit/knowledge/test_parse.py -q — 17 passed. New regressions cover XLSX and XLSM cell extraction, unchanged original source bytes, damaged worksheet rejection and rejection of an independently invalid modified timestamp.

• Focused Ruff check/format and mypy — passed.

• Local Codex CLI static review of the patch, full parser/tests and installed openpyxl reader/core implementation — completed with no actionable findings. Tests were run separately.

• make all RUN="uv run --no-sync python -m" PYTEST_JOBS=8 (ASCII path overrides for this Windows checkout) — passed: format/lint, mypy, and 4607 passed / 209 skipped backend tests.

• Normal Git pre-commit hook (not bypassed) — passed: format/lint/mypy, 24 passed affected tests, and TypeScript + Vite/PWA production build.

Remote CI is separate from local verification; no LLM or business document was used.

Target branch

• develop
• Regression tests and Unreleased CHANGELOG