Fix issues related to the `get_dofs_values` reshaping. Make slight improvement on Dofs gestion (_dof_per_nodes, dofs_positions).

Refactored `SolutionExplorer` to improve clarity and consistency in DoF computations, including changes to `_dof_per_nodes` logic and added support for position deduplication. Extended the `get_dofs_positions` method to handle duplicate removal and updated reshape logic for vector and tensor types in `get_dofs_values`. Added error handling for unsupported MixedElements and corresponding unit tests for better coverage.