add pytest warning file

Will throw a warning to remove show_plots after the cutoff date.

Author: schaubh

src/tests/test_show_plots_cutoff_notice.py

@@ -0,0 +1,34 @@
+#
+#  ISC License
+#
+#  Copyright (c) 2026, Autonomous Vehicle Systems Lab, University of Colorado at Boulder
+#
+#  Permission to use, copy, modify, and/or distribute this software for any
+#  purpose with or without fee is hereby granted, provided that the above
+#  copyright notice and this permission notice appear in all copies.
+#
+#  THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+#  WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+#  MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+#  ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+#  WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+#  ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+#  OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+#
+
+from datetime import date
+import warnings
+
+import conftest
+
+
+def test_show_plots_cutoff_notice():
+    """Emit a visible reminder after the cutoff date without failing tests."""
+    if date.today() <= conftest.SHOW_PLOTS_REMOVAL_DATE:
+        return
+
+    warnings.warn(
+        "The cutoff date for '--show_plots' has passed. Please remove the deprecated show_plots pytest option/fixture usage.",
+        UserWarning,
+        stacklevel=1,
+    )