Normalize maintenance database error handling

[Widthdom]

Summary

• unify vacuum, backfill-fold, optimize / index --optimize, and db integrity failures behind a versioned maintenance database classifier
• classify SQLite primary result codes and validation failures without depending on exception wording
• redact absolute paths and file URIs as a whole by default, preserve caller-spelled relative paths, and keep --show-paths as the explicit diagnostic opt-in
• preserve structured newer-schema, retry-exhausted lock, inaccessible-target, and optimize lock-holder diagnostics
• keep human and JSON recovery guidance specific to the actual failure category

Validation

• dotnet build CodeIndex.sln -c Release --no-restore -p:UseSharedCompilation=false (0 warnings, 0 errors)
• focused Issue Normalize maintenance error classification, redaction, and recovery hints #4856 and lock regression tests on .NET 8 / .NET 9: 25 passed per target framework
• related maintenance regression tests on .NET 8 / .NET 9: 52 passed, 1 environment-dependent trimmed-publish test skipped per target framework
• dotnet format CodeIndex.sln --verify-no-changes --no-restore --verbosity minimal
• dotnet run --project tools/CodeIndex.Changelog -- check
• git diff --check
• local cdidx status --check --json: index_matches_workspace: true
• full Release suite was also exercised before final focused validation; each target framework hit one unrelated intermittent failure, and both isolated failing tests passed when rerun

Adversarial review

Two Codex adversarial-review rounds were completed. Actionable findings around newer-schema and retry-exhausted lock preservation, whole-path redaction, index --optimize preamble leakage, inaccessible preflight targets, relative-path spelling, and lock-holder details were addressed with regression coverage.

Documentation and changelog

• updated the English and Japanese maintenance-error sections in USER_GUIDE.md
• updated the English and Japanese coverage notes in TESTING_GUIDE.md
• updated DEVELOPER_GUIDE.md
• added changelog.d/unreleased/4856.fixed.md

Related: #1526, #4338

Fixes #4856