Missing dedicated edge-case tests for exported _data() functions

[utkarshpawade]

The exported _data() functions are user-facing data preparation helpers but lack dedicated tests for edge cases like zero-length inputs, single observation, and single draw. Existing tests only exercise these functions implicitly through their plotting counterparts.

Functions: ppc_bars_data(), ppc_error_data(), ppc_scatter_data(), ppc_scatter_avg_data(), ppc_loo_pit_data(), ppd_data(), ppd_stat_data(), ppd_intervals_data()

Missing coverage:

• Zero-length inputs
• Single observation / single draw
• Alternative code paths (e.g., ppc_bars_data() with prob = 0, ppc_loo_pit_data() with custom bw)