Fix test_mmap_sparse.py to work on Windows, Mac, and Linux

[Copilot]

test_mmap_sparse.py passed on Linux/Mac but failed on Windows due to two incompatible patterns in Python's mmap on Windows.

Root causes

• Two mmap objects with different sizes on the same file handle raise OSError (WinError 87) — CreateFileMapping rejects a second mapping of a different size from the same handle.
• Writing via regular file I/O while a write-access (PAGE_READWRITE) mmap is active — conflicts with Windows file-mapping semantics.

Changes

• Separate "rb" file handle per mmap with explicit access=mmap.ACCESS_READ (PAGE_READONLY). Read-only mappings on Windows allow concurrent writes through other handles without sharing violations.
• Dedicated "r+b" write handle — mirrors the real-world pattern (background writer process / foreground reader process) and eliminates the same-handle conflict entirely.
• os.fsync() after each write — ensures the OS page cache is committed before a new mmap is created, needed for correctness on Windows.
• contextlib.ExitStack — manages all three handles and both mmap objects with safe, exception-proof cleanup.

# Before: single handle, default ACCESS_WRITE, two differently-sized mmaps — fails on Windows
with open(temp_file, "r+b") as f:
    mm1 = mmap.mmap(f.fileno(), len(non_sparse_data), offset=page_size)
    f.write(sparse_region_data)          # write while mmap active — problematic on Windows
    mm2 = mmap.mmap(f.fileno(), len(sparse_region_data), offset=0)  # WinError 87

# After: separate handles, ACCESS_READ, fsync — works on all platforms
with contextlib.ExitStack() as stack:
    read_f1 = stack.enter_context(open(temp_file, "rb"))
    read_f2 = stack.enter_context(open(temp_file, "rb"))
    write_f = stack.enter_context(open(temp_file, "r+b"))
    mm1 = stack.enter_context(mmap.mmap(read_f1.fileno(), len(non_sparse_data),
                                         access=mmap.ACCESS_READ, offset=page_size))
    write_f.write(sparse_region_data); write_f.flush(); os.fsync(write_f.fileno())
    mm2 = stack.enter_context(mmap.mmap(read_f2.fileno(), len(sparse_region_data),
                                         access=mmap.ACCESS_READ, offset=0))