fix: open gui not working (#4270)

* feat(core): add exec_file param to open_gui and improve execable lookup

- Add typed exec_file parameter to open_gui and document its behavior.
- Defer importing get_mapdl_path and only attempt to resolve the MAPDL
  executable when ansys.mapdl.core ATP helper is available.
- Use start_parm exec_file if provided, otherwise try get_mapdl_path; raise
  a clear MapdlRuntimeError if no executable path can be determined.
- Minor docstring/format tweaks.

* fix(core): propagate env_vars into start parameters, avoid circular import, and make MapdlGrpc._launch accept start_parm

- Add env_vars to start_parm in launch_mapdl so environment vars are carried in generated start parameters.
- Replace import from ansys.mapdl.core.launcher with package-level get_mapdl_path to prevent circular imports.
- Update MapdlGrpc._launch signature to accept an optional start_parm, use it when provided, and call _connect() without passing the port explicitly.

Author: germa89

doc/changelog.d/4270.fixed.md

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+Open gui not working