ENH: Fix mosaic plot plot_sagittal parameter warning

Fix mosaic plot `plot_sagittal` parameter warning: define the warning
message so that it can be tested exactly and catch the warning where
applicable.

Fixes:
```
nireports/tests/test_reportlets.py::test_mriqc_plot_mosaic[True-views24]
nireports/tests/test_reportlets.py::test_mriqc_plot_mosaic[True-views26]
  /home/runner/work/nireports/nireports/nireports/tests/test_reportlets.py:360:
   UserWarning: Argument ``plot_sagittal`` for plot_mosaic() should not be used.
      testfunc()
```

raised for example in:
https://github.com/nipreps/nireports/actions/runs/12681153218/job/35344304375#step:12:360

nireports/reportlets/mosaic.py

@@ -53,6 +53,8 @@
 )
 from nireports.tools.ndimage import load_api, rotate_affine, rotation2canonical
 
+plot_sagittal_msg = "Argument ``plot_sagittal`` for plot_mosaic() should not be used."
+
 
 def plot_segs(
     image_nii: ty.Union[str, nb.Nifti1Image],
@@ -546,7 +548,7 @@ def plot_mosaic(
         out_file = "mosaic.svg"
 
     if plot_sagittal and views[1] is None and views[0] != "sagittal":
-        warn("Argument ``plot_sagittal`` for plot_mosaic() should not be used.", stacklevel=2)
+        warn(plot_sagittal_msg, category=UserWarning, stacklevel=2)
         views = (views[0], "sagittal", None)
 
     # Create mask for bounding box

nireports/tests/test_reportlets.py

@@ -23,6 +23,7 @@
 """Test reportlets module."""
 
 import os
+import warnings
 from functools import partial
 from itertools import permutations
 from pathlib import Path
@@ -34,7 +35,7 @@
 from templateflow.api import get
 
 from nireports.reportlets.modality.func import fMRIPlot
-from nireports.reportlets.mosaic import plot_mosaic
+from nireports.reportlets.mosaic import plot_sagittal_msg, plot_mosaic
 from nireports.reportlets.nuisance import plot_carpet, plot_raincloud
 from nireports.reportlets.surface import cifti_surfaces_plot
 from nireports.reportlets.xca import compcor_variance_plot, plot_melodic_components
@@ -343,21 +344,33 @@ def test_mriqc_plot_mosaic(tmp_path, test_data_package, outdir, views, plot_sagi
 
     fname = f"mosaic_{'_'.join(v or 'none' for v in views)}_{plot_sagittal:d}.svg"
 
-    testfunc = partial(
-        plot_mosaic,
-        get("MNI152NLin6Asym", resolution=2, desc="LR", suffix="T1w"),
-        plot_sagittal=plot_sagittal,
-        views=views,
-        out_file=(outdir / fname) if outdir is not None else None,
-        title=f"A mosaic plotting example: views={views}, plot_sagittal={plot_sagittal}",
-        maxrows=5,
-    )
+    def _test_mriqc_plot_mosaic(outdir, views, plot_sagittal):
+        testfunc = partial(
+            plot_mosaic,
+            get("MNI152NLin6Asym", resolution=2, desc="LR", suffix="T1w"),
+            plot_sagittal=plot_sagittal,
+            views=views,
+            out_file=(outdir / fname) if outdir is not None else None,
+            title=f"A mosaic plotting example: views={views}, plot_sagittal={plot_sagittal}",
+            maxrows=5,
+        )
 
-    if views[0] is None or ((views[1] is None) and (views[2] is not None)):
-        with pytest.raises(RuntimeError):
+        if views[0] is None or ((views[1] is None) and (views[2] is not None)):
+            with pytest.raises(RuntimeError):
+                testfunc()
+        else:
             testfunc()
+
+    if plot_sagittal:
+        with warnings.catch_warnings():
+            warnings.filterwarnings(
+                "ignore",
+                message=plot_sagittal_msg,
+                category=UserWarning,
+            )
+            _test_mriqc_plot_mosaic(outdir, views, plot_sagittal)
     else:
-        testfunc()
+        _test_mriqc_plot_mosaic(outdir, views, plot_sagittal)
 
 
 def test_mriqc_plot_mosaic_2(tmp_path, test_data_package, outdir):