fix: surface BusinessReport appendix render failures

[shawcharles]

Closes #573.

Summary

This PR makes BusinessReport.full_report(include_appendix=True) visibly report when the estimator-native appendix cannot be rendered.

Previously, if the underlying estimator's summary() raised, full_report() caught the exception and silently omitted the Technical Appendix. The report could therefore look complete while missing the expected technical section.

The new behaviour keeps full_report() resilient while adding a bounded appendix-unavailable note. It covers failures while obtaining the estimator summary and while converting that appendix content to markdown text. It includes the exception class name, but intentionally does not include raw exception messages or tracebacks, since those may expose local paths, formulas, data values, or other internals.

Changes

• Preserve existing code-fenced Technical Appendix output when estimator summary() succeeds.
• Add a visible ## Technical Appendix fallback when appendix rendering fails.
• Add regression coverage for visible failure reporting, stringification failures, and non-disclosure of raw exception details.

Validation

• PYTHONPATH=. DIFF_DIFF_BACKEND=python pytest tests/test_business_report.py::TestAppendix -q
• PYTHONPATH=. DIFF_DIFF_BACKEND=python pytest tests/test_business_report.py -q
• PYTHONPATH=. DIFF_DIFF_BACKEND=python .venv-py39/bin/python -m pytest tests/test_business_report.py::TestAppendix -q
• python -m ruff check diff_diff/business_report.py tests/test_business_report.py
• git diff --check

[igerber]

@shawcharles this looks like a worthwhile contribution and solves a real issue you surfaced. Is it ready for review from your POV?

[shawcharles]

Yes, ready for review from my side.

I rebased the branch onto current upstream/main and reran the relevant checks:

• PYTHONPATH=. DIFF_DIFF_BACKEND=python pytest tests/test_business_report.py::TestAppendix -q
• PYTHONPATH=. DIFF_DIFF_BACKEND=python pytest tests/test_business_report.py -q
• PYTHONPATH=. DIFF_DIFF_BACKEND=python .venv-py39/bin/python -m pytest tests/test_business_report.py::TestAppendix -q
• git diff --check upstream/main...HEAD

I also moved the PR out of draft.

[shawcharles]

Follow-up: the current CI Gate failure is only the repository label gate:

The ready-for-ci label is required to run CI tests.

I do not have permission to apply that label, so from my side this is ready for maintainer review/CI whenever you want to label it.