fix(read): prevent panic on negative slice index in a key path

GetValue and Exists validated only the upper bound of a numeric key-path
segment, so a negative index such as "arr.-1" passed the check and indexed
the slice with -1, causing an index-out-of-range panic. Add a lower-bound
check (i < 0) to the []int, []string and []any cases in both functions.

read_test.go

@@ -499,3 +499,25 @@ func TestConfig_Duration_getter(t *testing.T) {
 	is.Eq(7*time.Second, c.Duration("missing", 7*time.Second))
 	is.Eq(time.Duration(0), c.Duration("missing"))
 }
+
+func TestConfig_GetValue_negativeIndex(t *testing.T) {
+	is := assert.New(t)
+
+	c := New("test-neg")
+	err := c.LoadData(map[string]any{
+		"arr": []any{"a", "b", "c"},
+	})
+	is.NoErr(err)
+
+	// A negative slice index in a key path must not panic; it is simply
+	// treated as not found.
+	is.NotPanics(func() {
+		v, ok := c.GetValue("arr.-1")
+		is.False(ok)
+		is.Nil(v)
+	})
+
+	is.NotPanics(func() {
+		is.False(c.Exists("arr.-1"))
+	})
+}