[MNT] Fix Tkinter UI crashes during test suite execution via headless Matplotlib backend

[Siddhazntx]

Reference Issues/PRs

Fixes #2140

What does this implement/fix? Explain your changes.

When running the test suite locally (specifically on Windows Conda environments or headless CI environments), tests that invoke model plotting (such as test_temporal_fusion_transformer.py::test_integration) cause the test suite to crash.

The crash occurs because self.plot_prediction() calls plt.subplots(). If the environment defaults to a GUI backend like TkAgg but lacks properly configured UI libraries, it attempts to open a physical window and fails.

Terminal Error Traceback

tests\test_models\test_baseline.py::test_integration[multiple_dataloaders_with_covariates0] ✓            57% █████▊   
 tests\test_models\test_temporal_fusion_transformer.py::test_integration[multiple_dataloaders_with_covariates0] ✓57% █████▊                                                                                                                        
 tests\test_models\test_baseline.py::test_integration[multiple_dataloaders_with_covariates1] ✓            57% █████▊   
 tests\test_models\test_temporal_fusion_transformer.py::test_integration[multiple_dataloaders_with_covariates1] ✓58% █████▊                                                                                                                        
 tests\test_models\test_baseline.py::test_integration[multiple_dataloaders_with_covariates2] ✓            58% █████▊   
 tests\test_models\test_temporal_fusion_transformer.py::test_integration[multiple_dataloaders_with_covariates2] ✓58% █████▊                                                                                                                        
 tests\test_models\test_baseline.py::test_integration[multiple_dataloaders_with_covariates3] ✓            58% █████▉   
 tests\test_models\test_temporal_fusion_transformer.py::test_integration[multiple_dataloaders_with_covariates3] ✓59% █████▉   
...
=============================================== short test summary info =============================================== 
SKIPPED [1] tests\test_metrics.py:446: CUDA is not available
SKIPPED [1] tests\test_models\test_temporal_fusion_transformer.py:274: Test skipped on Windows OS due to issues with ddp, see #1623
SKIPPED [2] tests\test_models\test_temporal_fusion_transformer.py:432: Test skipped on Win due to bug #1632, or if missing required packages
SKIPPED [1] tests\test_models\test_timexer.py:501: Skipping due to incompatibility with current model outputs.

Results (282.77s (0:04:42)):
     387 passed
       5 skipped

The Fix:

This PR updates tests/conftest.py to explicitly set matplotlib.use("Agg") at the very top of the file. This globally forces Matplotlib to use the headless Agg backend during pytest execution. Plots are now safely rendered in memory without attempting to launch physical GUI windows.

What should a reviewer concentrate their feedback on?

The placement of matplotlib.use("Agg") in conftest.py. It is placed at the top of the file to ensure it executes before any pytorch_forecasting modules are imported, which might otherwise prematurely initialize pyplot with the default backend.

PR checklist

• The PR title starts with either [ENH], [MNT], [DOC], or [BUG]. [BUG] - bugfix, [MNT] - CI, test framework, [ENH] - adding or improving code, [DOC] - writing or improving documentation or docstrings.
• Added/modified tests
• Used pre-commit hooks when committing to ensure that code is compliant with hooks. Install hooks with pre-commit install.
  To run hooks independent of commit, execute pre-commit run --all-files

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2142   +/-   ##
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  Coverage        ?   86.63%           
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  Files           ?      165           
  Lines           ?     9732           
  Branches        ?        0           
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  Hits            ?     8431           
  Misses          ?     1301           
  Partials        ?        0           

Flag	Coverage Δ
cpu	86.63% <100.00%> (?)
pytest	86.63% <100.00%> (?)

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