fix: update serialization test for new time_grid default behavior

- Fix test_multi_grids.jl to expect T_state instead of exception
- Update test to verify time_grid(sol) returns state grid by default
- Ensure consistency with new MultipleTimeGridModel default behavior

Author: ocots

test/suite/serialization/test_multi_grids.jl

@@ -234,8 +234,9 @@ function test_multi_grids()
                 successful=CTModels.successful(sol_unified),
             )
 
-            # time_grid without component should throw error
-            Test.@test_throws Exceptions.IncorrectArgument CTModels.time_grid(sol_multi)
+            # time_grid without component should return state grid (default behavior)
+            Test.@test CTModels.time_grid(sol_multi) == T_state
+            Test.@test CTModels.time_grid(sol_multi) == CTModels.time_grid(sol_multi, :state)
 
             # Invalid component should throw error
             Test.@test_throws Exceptions.IncorrectArgument CTModels.time_grid(