Fix swapped args in share_cube_with_user → _validate_cube_access (fixes #1901)

[egouilliard-leyton]

Summary

Fixes #1901.

`MOSCore.share_cube_with_user(cube_id, target_user_id)` is currently unusable: every well-formed call raises `ValueError("User '<cube_id>' does not exist or is inactive")` because the args passed to `_validate_cube_access` are in the wrong order.

This one-line fix passes the current user (`self.user_id`) plus the `cube_id`, matching both the validator's signature `_validate_cube_access(self, user_id, cube_id)` and the in-code comment immediately above the call ("Validate current user has access to this cube"). The target user's existence is already validated on the next line.

Diff

```diff
def share_cube_with_user(self, cube_id: str, target_user_id: str) -> bool:
...
# Validate current user has access to this cube

• self._validate_cube_access(cube_id, target_user_id)

• self._validate_cube_access(self.user_id, cube_id)

  Validate target user exists

  if not self.user_manager.validate_user(target_user_id):
  raise ValueError(...)

  return self.user_manager.add_user_to_cube(target_user_id, cube_id)
  ```

Test plan

• Add a `test_share_cube_with_user_success` unit test: owner creates a cube, adds a memory, shares with target, target's `search()` then returns the memory. (I can push this in a follow-up commit if a maintainer requests — the existing test layout in the repo would dictate the right location.)
• Add a `test_share_cube_with_user_denies_non_owner`: a user who does not own/have access to the cube cannot share it.
• Manual repro of the bug exists in the linked issue; the fix makes that repro pass.

Notes

• Reproduces deterministically on `b60616d` (HEAD of `main`, v2.0.18-dev) and `MemoryOS==2.0.17` on PyPI.
• I have not added a regression test in this PR because I wasn't sure which test directory layout the project prefers for `MOSCore` API tests. Happy to add one in a follow-up commit if a maintainer points to the right place.