refactor: use screenshot to show plots

[germa89]

Description

As the title. Using screenshot for all plot methods.
It also simplifies the code reducing duplicity.

Issue linked

Required by #4204

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 adds the "PLVA" command to the list of plot commands and refactors the plot handling logic to use the screenshot method consistently for all plot operations.

• Added "PLVA" to the PLOT_COMMANDS list
• Simplified plot command handling in the run method by replacing complex conditional logic with a single screenshot call
• Fixed control flow in the screenshot method by adding proper return statement

---

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

[codecov]

Codecov Report

❌ Patch coverage is 96.29630% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 91.29%. Comparing base (ec3cb1d) to head (834f743).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4196      +/-   ##
==========================================
- Coverage   91.31%   91.29%   -0.02%     
==========================================
  Files         193      193              
  Lines       15731    15710      -21     
==========================================
- Hits        14364    14343      -21     
  Misses       1367     1367              

🚀 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 😬