Address review on #849:
- Critical: add missing `await` on the exception-path fallback in
search_files_all -- it returned a coroutine instead of the result list.
Add unit tests for both the offset-page-raises (fallback) and
offset-zero-raises (propagate) paths, which previously had no coverage.
- Guard `_key` dedup against items missing both file_id and path (fall back
to id(item)) so they can't collapse under a shared None key and drop rows.
- Document the offset-ignored discard-and-refetch decision.
- Split the offset paging into `_search_offset_paged` (returns None to signal
fallback) and share the truncation warning via `_warn_if_truncated`,
cutting cognitive complexity below the threshold (SonarCloud S3776).
- Make the test side_effect helpers synchronous (SonarCloud S7503).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
