Close cached readers when exiting an open_files context

[betacatsling]

Exiting an open_files context leaves simplecache and filecache readers open because open_many() returns handles without populating the OpenFile wrappers. Close those handles on exit, including when the consumer raises an exception.

Regression coverage exercises both cache implementations, normal and exceptional exits, and cold and warm cache reads. The four regressions fail on the unchanged base and pass with the fix. Local validation: cached-filesystem and core suites 137 passed, 37 skipped; Ruff lint/format and git diff --check passed. Remote backends and downstream suites were not run.

AI assistance was used for implementation and local verification.

Fixes #2130.