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_test.go

@@ -9,6 +9,7 @@ import (
 
 	"github.com/TecharoHQ/anubis/internal/dns"
 	"github.com/TecharoHQ/anubis/lib/store/memory"
+	"github.com/google/cel-go/cel"
 	"github.com/google/cel-go/common/types"
 	"github.com/google/cel-go/common/types/ref"
 )
@@ -688,6 +689,14 @@ func TestNewEnvironment(t *testing.T) {
 			description:   "should return values in correct range",
 			shouldCompile: true,
 		},
+		{
+			name:          "randInt-large-bound",
+			expression:    `randInt(2147483647) >= 0`,
+			variables:     map[string]any{},
+			expectBool:    boolPtr(true),
+			description:   "should accept int32-max bounds without overflow",
+			shouldCompile: true,
+		},
 		{
 			name:          "strings-extension-size",
 			expression:    `"hello".size() == 5`,
@@ -750,3 +759,65 @@ func TestNewEnvironment(t *testing.T) {
 func boolPtr(b bool) *bool {
 	return &b
 }
+
+func TestRandIntInvalidBounds(t *testing.T) {
+	env, err := New(cel.Variable("contentLength", cel.IntType))
+	if err != nil {
+		t.Fatalf("failed to create environment: %v", err)
+	}
+
+	tests := []struct {
+		name        string
+		expression  string
+		variables   map[string]any
+		wantErrText string
+		description string
+	}{
+		{
+			name:        "zero-bound-literal",
+			expression:  `randInt(0)`,
+			variables:   map[string]any{},
+			wantErrText: "randInt bound must be positive",
+			description: "randInt(0) should return a CEL error, not panic",
+		},
+		{
+			name:        "negative-bound-literal",
+			expression:  `randInt(-5)`,
+			variables:   map[string]any{},
+			wantErrText: "randInt bound must be positive",
+			description: "randInt(-5) should return a CEL error, not panic",
+		},
+		{
+			name:        "zero-bound-from-variable",
+			expression:  `randInt(contentLength)`,
+			variables:   map[string]any{"contentLength": 0},
+			wantErrText: "randInt bound must be positive",
+			description: "attacker-controlled zero contentLength should error gracefully",
+		},
+		{
+			name:        "negative-bound-from-variable",
+			expression:  `randInt(contentLength)`,
+			variables:   map[string]any{"contentLength": -1},
+			wantErrText: "randInt bound must be positive",
+			description: "attacker-controlled negative contentLength should error gracefully",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			prog, err := Compile(env, tt.expression)
+			if err != nil {
+				t.Fatalf("failed to compile expression %q: %v", tt.expression, err)
+			}
+
+			result, _, err := prog.Eval(tt.variables)
+			if err == nil {
+				t.Fatalf("%s: expected an evaluation error, got result %v", tt.description, result)
+			}
+
+			if !strings.Contains(err.Error(), tt.wantErrText) {
+				t.Errorf("%s: expected error containing %q, got %q", tt.description, tt.wantErrText, err.Error())
+			}
+		})
+	}
+}