Add plot_ppc_intervals

[aloctavodia]

We should have a plot similar to https://mc-stan.org/bayesplot/reference/PPC-intervals.html

[PiyushPanwarFST]

@aloctavodia
Could you please assign it to me?

[PiyushPanwarFST]

Here’s my proposed approach for implementing plot_ppc_intervals.
Let me know if there’s a better way or if I’m missing anything important.

Step 1: Input Handling

• Accept InferenceData (observed_data, posterior_predictive).
• Params: prob, prob_outer, optional x, plus var_names, coords, backend, visuals, ax.

Step 2: Core Logic

• Extract posterior_predictive arrays per observation.
• Compute median + quantiles via numpy.quantile for inner/outer intervals.
• Apply var_names/coords/x filtering and prepare coordinates for points + intervals.

Step 3: Visualization

• Use existing functions in visuals (e.g., visuals.scatter_xy) to draw points and intervals.

[aloctavodia]

Use plot_forest and this example as a guide for both the logic and the argument names/signature. See for instance that operations are vectorized and that we don't use prob, prob_outer argument but ci_probs