fix(nimbus): capture branch feature value changes in history changelog

Because

* NimbusBranchesForm.save() called super().save() which triggered
  NimbusChangeLogFormMixin.save() to generate the changelog snapshot
  before self.branches.save() ran, causing the changelog to capture
  stale feature values instead of the updated ones
* Feature value changes were invisible on the History page

This commit

* Moves self.branches.save() before super().save() in
  NimbusBranchesForm so that branch feature values are persisted
  before the changelog snapshot is taken
* Adds a test that saves an experiment with initial feature values,
  updates the values, and asserts the changelog captures the change

Fixes #14998

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jaredlockhart

experimenter/experimenter/nimbus_ui/forms.py

@@ -829,8 +829,8 @@ def clean(self):
 
     @transaction.atomic
     def save(self, *args, **kwargs):
-        experiment = super().save(*args, **kwargs)
         self.branches.save()
+        experiment = super().save(*args, **kwargs)
 
         if experiment.is_rollout:
             branches = experiment.branches.all()

experimenter/experimenter/nimbus_ui/tests/test_forms.py

@@ -3226,6 +3226,18 @@ def test_branches_form_saves_branches(self):
 
         changelog = experiment.changes.get()
         self.assertIn("updated branches", changelog.message)
+        changelog_branches = changelog.experiment_data["branches"]
+        changelog_fv_values = [
+            fv["value"] for b in changelog_branches for fv in b["feature_values"]
+        ]
+        self.assertIn(
+            json.dumps({"control-feature1-key": "control-feature-1-value"}),
+            changelog_fv_values,
+        )
+        self.assertIn(
+            json.dumps({"treatment-feature-1-key": "treatment-feature-1-value"}),
+            changelog_fv_values,
+        )
 
     def test_branches_form_saves_added_feature_config(self):
         application = NimbusExperiment.Application.DESKTOP