fix: guard heatmap non-unique index test against single-row DataFrames

When the frequency DataFrame has only one row (e.g., due to sample
query filtering), the "contig" column is unique, so passing
index="contig" does not raise ValueError. Only assert the error when
the contig values are actually non-unique.

This fixes a flaky test failure in
test_gene_cnv_frequencies_with_str_cohorts_and_sample_query.

Author: khushthecoder

tests/anoph/test_frq.py

@@ -16,7 +16,7 @@ def check_plot_frequencies_heatmap(api, frq_df):
     # Test index parameter - if None, should use dataframe index.
     fig = api.plot_frequencies_heatmap(frq_df, show=False, index=None, max_len=None)
 
-    if "contig" in list(frq_df.columns):
+    if "contig" in list(frq_df.columns) and not frq_df["contig"].is_unique:
         # Not unique.
         with pytest.raises(ValueError):
             api.plot_frequencies_heatmap(