Add test case for APPEND command usage on integer value (#12429)

Add test coverage to validate object encoding update on APPEND command usage on a integer value

tests/unit/type/string.tcl

@@ -656,4 +656,19 @@ if {[string match {*jemalloc*} [s mem_allocator]]} {
            }
         }
     }
+
+    test {APPEND modifies the encoding from int to raw} {
+        r del foo
+        r set foo 1
+        assert_encoding "int" foo
+        r append foo 2
+
+        set res {}
+        lappend res [r get foo]
+        assert_encoding "raw" foo
+        
+        r set bar 12
+        assert_encoding "int" bar
+        lappend res [r get bar]
+    } {12 12}
 }