Use modelbased posterior predictions in check_predictions() for Bayesian models

[Copilot]

check_predictions() still routed stanreg/brmsfit models through bayesplot::pp_check() and then reshaped the result into a special Stan-specific format. This updates the Bayesian path to use the easystats prediction stack directly so Bayesian models produce the same posterior predictive-check data shape and plotting behavior as other supported models.

• Bayesian prediction path

  • Replace the bayesplot::pp_check() branch in check_predictions.stanreg() with modelbased::estimate_prediction(..., keep_iterations = TRUE).
  • Convert retained posterior draws from iter_* to the standard sim_* columns used elsewhere by check_predictions().
  • Preserve existing metadata (response_name, model_info, type, bandwidth, check_range) and back-transform simulated draws when the response is transformed.

• Output contract

  • Remove the Stan-specific discrete reshaping (x / Group / Count with is_stan = TRUE).
  • Return Bayesian posterior predictive data in the same wide format as other methods: replicated draws plus observed response.

• Coverage and docs

  • Add focused regression tests for continuous and discrete Bayesian models to assert the standardized output shape and attributes.
  • Update package version, NEWS entry, and check_predictions() documentation to reflect the new Bayesian implementation.

Example of the new Bayesian data contract:

out <- check_predictions(model, iterations = 5)

names(out)
# [1] "sim_1" "sim_2" "sim_3" "sim_4" "sim_5" "y"

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