Raise IsADirectoryError when opening an implicit directory in MemoryFileSystem

[itzzdev09]

_open only rejected paths in pseudo_dirs (explicit directories created via makedirs), so a path that is a directory only because files live under it — the common case after pipe/touch — could be silently opened as a file, shadowing everything beneath it:

m.pipe('/dir/file', b'data')
m.open('/dir', 'wb')   # used to succeed silently
m.glob('/dir/**')      # -> [] afterwards, file now unreachable
m.head('/dir', 10)     # -> FileExistsError, contradicting what open() just allowed

Fix mirrors the directory check info() already does, and only checks the store (not isdir) so it doesn't add an extra call on the common put/pipe path.

Added a regression test (test_open_implicit_directory_raises) covering rb/wb/ab; confirmed it fails on master without the fix.

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