Refetch mmap blocks when recreating a missing backing file

[adenzhou1350]

A named MMapCache recreates its backing file when it no longer exists, but previously retained the old set of cached blocks. Restoring a pickled cache after deleting that file therefore returned zero-filled bytes instead of fetching the original data. The same condition is reachable when reopening an HTTP reader with the saved location/blocks cache options.

Clear the block set in place whenever a new backing file is created. Existing files keep their cached blocks; keeping the same set object preserves shared bookkeeping. Read bounds and prefetch behavior are unchanged.

Validation on current master (94afb3f6):

• Four new regression/control cases: 3 fail and 1 passes before, all pass after; includes a real loopback HTTP reader.
• Seven core/local/memory/cache test files, Linux Python 3.12: 637 passed, 39 skipped (optional providers and Windows-specific cases).
• Three additional real Windows Python 3.12 mmap/pickle controls: 1/3 passes before, 3/3 after.
• Repository-pinned Ruff 0.14.3 check/format and diff check pass.

This is not full CI or cloud-provider qualification. AI-assisted implementation and tests; results were checked against the unmodified source. It does not address serialization of an HTTP-bound fetcher itself.