fix(expressions): validate randInt bounds before rand.IntN

Non-positive or platform-overflowing arguments to the CEL randInt
helper used to reach rand.IntN unchecked, surfacing a CEL evaluator
error during request processing when policies passed
attacker-influenced values (e.g. contentLength). Reject non-positive
bounds and detect int narrowing explicitly, returning a typed CEL
error in both cases.

Ref: AWOO-010
Signed-off-by: Xe Iaso <me@xeiaso.net>

lib/policy/expressions/environment.go

@@ -222,7 +222,16 @@ func New(opts ...cel.EnvOption) (*cel.Env, error) {
 						return types.ValOrErr(val, "value is not an integer, but is %T", val)
 					}
 
-					return types.Int(rand.IntN(int(n)))
+					if n <= 0 {
+						return types.NewErr("randInt bound must be positive, got %d", int64(n))
+					}
+
+					bound := int(n)
+					if types.Int(bound) != n {
+						return types.NewErr("randInt bound %d overflows platform int", int64(n))
+					}
+
+					return types.Int(rand.IntN(bound))
 				}),
 			),
 		),