Address failing tests on pyside6

[nstelter-slac]

Describe the bug

tests failing consistently on master are currently:

pydm/tests/test_plugins_import.py
pydm/tests/widgets/test_multiaxis_plot.py
pydm/tests/widgets/test_baseplot.py
pydm/tests/data_plugins/test_plugin.py

test failures might be bugs or related to unimplemented features (like designer support atm)

Steps to reproduce

1. create conda env with python 3.13 and pyside6:

  $ conda create -n pydm-environment-pyside python pip numpy scipy six psutil pyqtgraph pydm -c conda-forge
  $ source activate pydm-environment
  $ pip install PySide6 # or can install with conda in earlier cmd

2. activate conda pyside6 env and export QT_API=pyside6

3. run all the tests with python run_tests.py, and individually with pytest pydm/tests/<test_name>.py
   (note: you need to ignore 'pydm/tests/test_plugins_import.py' in run_tests.py if want to see results of other tests after, since it's failure kills the testing)

Expected behavior

all tests should be passing with pyside6 (or disable if not relevant for pyside6)

determine if each test is failing from bugs or missing feature implementation, and fix accordingly or note that waiting for missing feature.

tests can be potentially disabled if not relevant to pyside6

Operating System.

Linux

OS Version Details.

No response

Python Version

3.13.5

Packages Version

Additional context.

No response