Revert "SERVER-84420: Remove unnecessary tasserts."

This reverts commit dd42977758.

src/mongo/db/pipeline/group_processor_base.h

@@ -132,6 +132,7 @@ public:
      * Should not be used once execution has begun.
      */
     std::vector<boost::intrusive_ptr<Expression>>& getMutableIdExpressions() {
+        tassert(7020503, "Can't mutate _id fields after initialization", !_executionStarted);
         return _idExpressions;
     }
 
@@ -147,6 +148,8 @@ public:
      * Should not be used once execution has begun.
      */
     std::vector<AccumulationStatement>& getMutableAccumulationStatements() {
+        tassert(
+            7020504, "Can't mutate accumulated fields after initialization", !_executionStarted);
         return _accumulatedFields;
     }