Fix FileNotFoundError when clear_cache() races with separate_files=True

[lode-braced]

Closes #399

Problem

With the pickle backend and separate_files=True, _PickleCore._clear_all_cache_files() lists the cache directory and then removes each matching file. If two clear_cache() calls run concurrently (same wrapper from two threads, or separate wrappers/processes sharing a cache_dir), both can list the same file. One removes it; the other's os.remove() then raises FileNotFoundError, aborting that clear before it processes the remaining files.

The existing retry loop only handles PermissionError (Windows file locking), so the missing-file case propagates.

Fix

Treat FileNotFoundError from the per-file os.remove() as "already cleared" and continue with the next file. The handler wraps only the individual deletion. PermissionError retry behavior and propagation of other errors are unchanged. No new locking is introduced.

This does not address in-flight computations re-creating entries after a clear; that is a separate invalidation concern.

Tests

Adds test_concurrent_clear_cache_separate_files, parametrized for a shared wrapper and for separate wrappers on the same directory. It uses real cache files and real deletions, and coordinates scheduling deterministically: both callers list the directory before either deletes, and the second deletion of each file is attempted only after the first completes. It asserts that both clears succeed, that all matching files are removed, and that unrelated files are untouched. The test fails on the current master and passes with this change.