Preserve local objects when downloading onto the source

[zhechen-cs]

Description

Handle shutil.SameFileError before ordinary copy failures in the local storage driver. Downloading an object onto itself still returns False, but no longer deletes the source object during failure cleanup.

The regression checks both an explicit source filename and its containing directory with a trailing separator, then verifies the stored contents and object metadata remain available.

Fixes #2200

Status

Ready for review.

Validation

• Local storage and base storage tests: 33 passed, 2 subtests passed (Python 3.12).
• Both regression cases fail against unmodified trunk with FileNotFoundError after the source is deleted.
• Black, isort, flake8, pylint and git diff --check pass for the changed files.
• Live cloud integration tests were not run; this fix only affects the local storage driver.

Checklist

• Code linting
• Documentation (no public API change)
• Tests
• ICLA (small bug fix)