feat: add STATUS enum for MAPDL state management

[germa89]

Description

Use enum instead of just strings.

Issue linked

NA

Checklist

• I have visited and read Develop PyMAPDL.
• I have tested my changes locally
• I have added necessary documentation or updated existing documentation.
• I have followed the PyMAPDL coding style guidelines.
• I have added appropriate unit tests that also ensure the minimal coverage criteria.
• I have reviewed my changes before submitting this pull request.
• I have linked the issue or issues that are solved to the PR if any.
• I have assigned this PR to myself.
• I have marked this PR as draft if it is not ready to be reviewed yet.
• I have made sure that the title of my PR follows Commit naming conventions (e.g. feat: adding new MAPDL command)

[Copilot]

Pull Request Overview

This PR introduces a STATUS enum to replace string literals for MAPDL state management, improving type safety and code maintainability. The changes move from magic strings like "exited", "exiting", and "OK" to well-defined enum values.

• Adds a STATUS enum with EXITED, EXITING, and RUNNING values
• Updates the check_status property to use enum values instead of strings
• Improves type annotation by specifying STATUS as the return type

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.31%. Comparing base (b3a026c) to head (a23667a).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4221      +/-   ##
==========================================
- Coverage   91.39%   91.31%   -0.08%     
==========================================
  Files         193      193              
  Lines       15715    15720       +5     
==========================================
- Hits        14362    14355       -7     
- Misses       1353     1365      +12     

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[germa89]

@pyansys-ci-bot LGTM

[pyansys-ci-bot]

✅ Approving this PR because germa89 said so in here 😬