fix(redis): never send a zero or shortened expiry

Set and SetMany sent PSETEX with 0 for a TTL under 1ms, which Redis
rejects. They sent SETEX with truncated seconds for a fractional TTL,
so keys expired early. A shared helper now rounds up to a millisecond
and uses SETEX only for whole seconds.

internal/cache/backends/redis_ttl_bounds_test.go

@@ -0,0 +1,45 @@
+package backends
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestRedisSetTTLNeverZeroOrEarly(t *testing.T) {
+	mr := NewMiniredisServer(t)
+	b, err := NewRedisBackend(DefaultRedisConfig(mr.GetAddr()))
+	require.NoError(t, err)
+	defer b.Close()
+	ctx := context.Background()
+
+	t.Run("sub-millisecond ttl is accepted", func(t *testing.T) {
+		require.NoError(t, b.Set(ctx, "sub", []byte("v"), 500*time.Microsecond))
+	})
+	t.Run("sub-millisecond ttl accepted by SetMany", func(t *testing.T) {
+		require.NoError(t, b.SetMany(ctx, map[string][]byte{"sub-many": []byte("v")}, 500*time.Microsecond))
+	})
+	t.Run("fractional seconds are not rounded down", func(t *testing.T) {
+		require.NoError(t, b.Set(ctx, "frac", []byte("v"), 1999*time.Millisecond))
+		_, ttl, ok, err := b.Get(ctx, "frac")
+		require.NoError(t, err)
+		require.True(t, ok)
+		require.Greater(t, ttl, time.Second, "1999ms ttl must not be stored as 1s")
+	})
+	t.Run("fractional seconds SetMany", func(t *testing.T) {
+		require.NoError(t, b.SetMany(ctx, map[string][]byte{"frac-many": []byte("v")}, 1999*time.Millisecond))
+		_, ttl, ok, err := b.Get(ctx, "frac-many")
+		require.NoError(t, err)
+		require.True(t, ok)
+		require.Greater(t, ttl, time.Second)
+	})
+	t.Run("whole seconds still work", func(t *testing.T) {
+		require.NoError(t, b.Set(ctx, "whole", []byte("v"), 5*time.Second))
+		_, ttl, ok, err := b.Get(ctx, "whole")
+		require.NoError(t, err)
+		require.True(t, ok)
+		require.Greater(t, ttl, 3*time.Second)
+	})
+}