Reject invalid memory filesystem copy destinations

[dlowzzxx]

Summary

MemoryFileSystem.cp_file inserts directly into its store without validating the destination hierarchy. If /parent is a file, copy("/src", "/parent/child") still succeeds. Afterwards info("/parent") reports a directory while isfile("/parent") is true. mv also removes the original source after creating this invalid destination.

Direct file-to-directory and directory-to-file copies can similarly leave conflicting entries in store and pseudo_dirs.

Validate destination ancestors and the exact destination before changing either collection. The ancestor check uses the same FileExistsError behavior as MemoryFileSystem._open. Ordinary overwrites, missing destination parents, public copy-into-directory behavior, and idempotent directory copies remain supported.

Tests

Regression tests cover cp_file, copy, and mv with file ancestors; explicit and implicit directory destinations; directory-to-file collisions; and unchanged source/destination contents after rejection. Success cases cover normal overwrites and directory copies.

Validation

• Memory filesystem module: 85 passed.
• Memory, filesystem-specification, utility, and core suites: 538 passed, 129 skipped, 1 xfailed (Windows, Python 3.13).
• Ruff 0.14.3 check and format check; git diff --check.
• The broader run emits a Windows socket-server cleanup warning in an existing test_walk_on_error_reaches_subdirectories test; no tests fail.

Checking an implicit destination directory scans the memory store, as existing directory detection does. No remote requests or external services are needed for the new tests.

Assisted-by: OpenAI Codex. The failure was reproduced locally and the code and validation were reviewed before submission.