Add arviz_version to meridian_model.

PiperOrigin-RevId: 842871340

Author: ronellecaguioa

proto/CHANGELOG.md

@@ -23,6 +23,8 @@ To release a new version (e.g. from `1.0.0` -> `2.0.0`):
 
 ## [Unreleased]
 
+* Add field to record ArviZ version in meridian_model proto definition.
+
 ## [1.1.0] - 2025-12-08
 
 * Add proto definitions needed for Meridian Scenario Planner.

proto/mmm/v1/model/meridian/meridian_model.proto

@@ -582,6 +582,9 @@ message MeridianModel {
   // The semantic version of the Meridian library used to generate this model.
   string model_version = 2 [features.field_presence = EXPLICIT];
 
+  // The ArviZ version used to serialize inference data.
+  string arviz_version = 20 [features.field_presence = EXPLICIT];
+
   Hyperparameters hyperparameters = 3;
   PriorDistributions prior_distributions = 11 [deprecated = true];
   PriorTfpDistributions prior_tfp_distributions = 18;