Add _get_canonical_metric_name() to DatasetValidator that searches only
available_metrics, preventing a column with matching case-insensitive name
from shadowing a saved metric's canonical casing.
Update all 7 chart plugins (xy, table, pie, big_number, handlebars,
mixed_timeseries, pivot_table) to branch on saved_metric flag: saved
metrics now go through _get_canonical_metric_name while regular column
refs continue to use _get_canonical_column_name.
Fix pre_validate alias handling in xy and mixed_timeseries plugins to
accept Pydantic AliasChoices keys (metrics/x_axis/metrics_b) so payloads
using canonical Superset field names are not incorrectly rejected.
Add TestGetCanonicalMetricName, TestSavedMetricNormalizationCorrectness,
and TestPreValidateAliasHandling test classes covering the collision case
and alias acceptance.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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