Fix MemoryFileSystem._internal_path prefix stripping corruption

[rootkiller6788]

Problem

MemoryFileSystem._internal_path stripped the /mem/ prefix with:

return '/' + resolve_path(path).lstrip(self._prefix)

str.lstrip treats its argument as a set of characters, not a literal prefix. So any leading m or e characters after the prefix were also stripped, corrupting path components:

Input	Old internal path
/mem/models/a	/odels/a
/mem/eval/b	/val/b
/mem/memory	/ory

This is observable via listdir (fs.mkdirs('/mem/models/a') then fs.listdir('/mem') returns ['odels'] instead of ['models']) and causes distinct paths such as /mem/models and /mem/odels to collide.

Fix

Replace lstrip with an explicit startswith check plus a slice, and keep mapping the bare prefix /mem to the root:

def _internal_path(self, path):
  path = resolve_path(path)
  if path.startswith(self._prefix):
    path = path[len(self._prefix):]
  elif path == self._prefix.rstrip('/'):
    path = ''
  return '/' + path

Tests

Added MemoryFileSystemTest.test_internal_path_prefix_stripping, which verifies that m/e-prefixed components are preserved, and that /mem/models and /mem/odels remain distinct.