test(ultracode): harden cross-platform CI coverage

[yishuiliunian]

Summary

Harden the Ultracode cross-platform test suite so the workflow remains reliable on Unix and Windows.

Changes

• Cover replacement retry delay, capped backoff, and non-retryable verification paths in storage.
• Scope the synthetic cleanup-failure fixture to Unix where its filesystem semantics are valid, and cover Windows locked-file cleanup behavior with native handles.
• Give the backend grace-shutdown test scheduling headroom while preserving terminal-state assertions.
• Make the unresolved-secret effect fixture shell-independent with a cross-platform exit 0 command, avoiding platform-specific redirection and stdout-pipe behavior.

Validation

Focused runtime, storage, backend, clippy, rustfmt, and diff checks pass locally. The prior CI run was canceled after six checks passed but the Windows aggregate test remained in progress far beyond its historical runtime; this head reruns the Windows fixture with the pipe-independent command.