When building from `SampleYAML`, the sample index column is not cast to `DataType::String` [potential BUG]

When building from SampleYAML, the sample index column is not cast to DataType::String like the CSV path is. If the YAML parses the index column as a non-string type (e.g. numbers), later lookups like get_sample(name: &str) can fail due to a dtype mismatch. Consider applying the same .lazy().with_column(col(final_index.clone()).cast(DataType::String)).collect()? normalization used in the CSV branch.

                let df = resolve_yaml_to_dataframe(&yaml_path)?
                    .lazy()
                    .with_column(col(final_index.clone()).cast(DataType::String))
                    .collect()?;

Originally posted by @Copilot in https://github.com/khoroshevskyi/peprs/pull/43#discussion_r3104373679

Find the code path that builds a dataframe from SampleYAML and compare it with the CSV branch, which already normalizes the index column. Apply the same index-string normalization shown in the issue, then verify that a numeric YAML index can be used by get_sample(name: &str).