Lists all available Biomart marts (databases) from Ensembl.
Biomart organizes biological data in a hierarchy: MART -> DATASET -> ATTRIBUTES/FILTERS.
This function returns all available marts as a CSV string.
Returns:
str: CSV-formatted table of all marts with their display names and descriptions.
Example:
list_marts()
>>> "name,display_name,description
ENSEMBL_MART_ENSEMBL,Ensembl Genes,Gene annotation from Ensembl
ENSEMBL_MART_MOUSE,Mouse strains,Strain-specific data for mouse
..."