fix(export): report incomplete protected backups

[oleksii-tumanov]

What does this PR do?

Fixes #3769.

An incomplete protected backup now warns which files failed and returns None, instead of reporting success. Other files are still copied, and the caller can continue with its overwrite.

Same-day reuse checks every selected artifact so an unchanged graph cannot hide a failed sidecar copy on retry. Comparison errors still allow a copy attempt, and closed output streams do not change the backup result. Eligibility and daily retention are unchanged.

Type of change

• Bug fix
• New feature
• Documentation
• Tests or CI
• Refactor
• Security fix

How was this tested?

The new regressions fail on the base and pass with this change. A read-only backup-file reproduction now reports failure without printing success.

pytest tests/test_export.py -k backup -q
17 passed

pytest tests/test_export.py tests/test_watch.py tests/test_atomic_writes.py -q
268 passed, 3 skipped

ruff check .
All checks passed

The full suite ran: 5,927 passed, 15 skipped, 16 failed because the optional Erlang/R/Solidity/VB.NET grammar dependencies could not be downloaded. The same 16 failures reproduce on the unmodified base. All five skillgen guards and graphify update . completed.

Graphify-specific checklist

• I added or updated tests for behavior changes.
• I updated documentation or confirmed that no documentation is needed.
• I updated generated skill artifacts when changing their source fragments. (Not applicable; no fragments changed.)
• I considered compatibility across supported Python versions.
• I confirmed that no API keys, generated graph data, or local-only files are included.

[oleksii-tumanov]

This complements #2965, which reports what changed when a forced update shrinks the graph. This PR covers backup failures instead. The fixes are independent, and I'd appreciate a review of #2965 too.