Lazy-import notebook test deps to fix conda-build collection

nest_asyncio is a dev-only dependency and not part of the conda runtime
deps, so importing it at module level broke pytest collection in the
conda-build pipeline. Move nbformat, nest_asyncio and ExecutePreprocessor
imports into _run_notebook so collection no longer requires them. ✨

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com> 🍐

Author: anawas

karabo/test/test_notebooks.py

@@ -1,22 +1,26 @@
 import os
 
-import nbformat
-import nest_asyncio
 import pytest
-from nbconvert.preprocessors import ExecutePreprocessor
 
 from karabo.test.conftest import IS_GITHUB_RUNNER
 from karabo.util.plotting_util import Font
 
-nest_asyncio.apply()
-
 RUN_NOTEBOOK_TESTS = os.environ.get("RUN_NOTEBOOK_TESTS", "false").lower() == "true"
 
 # get notebook-dir not matter cwd
 notebook_dir = os.path.join(os.path.dirname(os.path.dirname(__file__)), "examples")
 
 
 def _run_notebook(notebook: str) -> None:
+    # Imported lazily: these are dev-only dependencies (see pyproject.toml) and
+    # are not part of the conda runtime deps, so importing them at module level
+    # would break test collection in the conda-build pipeline.
+    import nbformat
+    import nest_asyncio
+    from nbconvert.preprocessors import ExecutePreprocessor
+
+    nest_asyncio.apply()
+
     notebook = os.path.join(notebook_dir, notebook)
     print(Font.BOLD + Font.BLUE + "Testing notebook " + notebook + Font.END)