test(budget): add fail-open error behavior and ResetDay validation tests

- Document intentional fail-open: DB errors in budget checks don't block AI steps
- Test rolling mode rejects ResetDay outside 1-28
- Test calendar mode silently clamps invalid ResetDay to 1

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: michaelmcnees

internal/budget/budget_test.go

@@ -2,6 +2,7 @@ package budget_test
 
 import (
 	"context"
+	"fmt"
 	"testing"
 	"time"
 
@@ -105,6 +106,50 @@ func TestChecker_Check_AllPass(t *testing.T) {
 	assert.False(t, result.Warning)
 }
 
+func TestChecker_Check_FailOpenOnGetTotalUsageError(t *testing.T) {
+	checker := &budget.Checker{
+		GlobalMonthlyTokenLimit: 1000,
+		ResetMode:               "calendar",
+		ResetDay:                1,
+		GetTotalUsage: func(ctx context.Context, period time.Time) (int64, error) {
+			return 0, fmt.Errorf("db connection lost")
+		},
+	}
+
+	result := checker.Check(context.Background(), budget.CheckInput{
+		TeamID:   "team-1",
+		Provider: "openai",
+	})
+
+	// Intentional fail-open: DB errors should not block AI steps.
+	assert.False(t, result.Blocked)
+	assert.False(t, result.Warning)
+}
+
+func TestChecker_Check_FailOpenOnGetTeamBudgetError(t *testing.T) {
+	checker := &budget.Checker{
+		ResetMode: "calendar",
+		ResetDay:  1,
+		GetTotalUsage: func(ctx context.Context, period time.Time) (int64, error) {
+			return 0, nil
+		},
+		GetProviderUsage: func(ctx context.Context, teamID, provider string, period time.Time) (int64, error) {
+			return 0, nil
+		},
+		GetTeamBudget: func(ctx context.Context, teamID, provider string) (*budget.TeamBudget, error) {
+			return nil, fmt.Errorf("db timeout")
+		},
+	}
+
+	result := checker.Check(context.Background(), budget.CheckInput{
+		TeamID:   "team-1",
+		Provider: "openai",
+	})
+
+	assert.False(t, result.Blocked)
+	assert.False(t, result.Warning)
+}
+
 func TestChecker_CheckExecutionBudget(t *testing.T) {
 	result := budget.CheckExecutionBudget(50000, 50001)
 	assert.True(t, result.Blocked)

internal/config/config_test.go

@@ -206,3 +206,32 @@ func TestLoad_BudgetDefaults(t *testing.T) {
 	assert.Equal(t, int64(0), cfg.Engine.Budget.GlobalMonthlyTokenLimit)
 	assert.Equal(t, int64(0), cfg.Engine.Budget.DefaultTeamMonthlyTokenLimit)
 }
+
+func TestLoad_BudgetResetDay_RollingInvalid(t *testing.T) {
+	// ResetDay 0 with rolling mode should error
+	t.Setenv("MANTLE_ENGINE_BUDGET_RESET_MODE", "rolling")
+	t.Setenv("MANTLE_ENGINE_BUDGET_RESET_DAY", "0")
+
+	cmd := newTestCommand()
+	_, err := Load(cmd)
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "reset_day must be between 1 and 28")
+
+	// ResetDay 29 with rolling mode should error
+	t.Setenv("MANTLE_ENGINE_BUDGET_RESET_DAY", "29")
+	cmd = newTestCommand()
+	_, err = Load(cmd)
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "reset_day must be between 1 and 28")
+}
+
+func TestLoad_BudgetResetDay_CalendarClamps(t *testing.T) {
+	// Invalid ResetDay with calendar mode should silently clamp to 1
+	t.Setenv("MANTLE_ENGINE_BUDGET_RESET_MODE", "calendar")
+	t.Setenv("MANTLE_ENGINE_BUDGET_RESET_DAY", "0")
+
+	cmd := newTestCommand()
+	cfg, err := Load(cmd)
+	require.NoError(t, err)
+	assert.Equal(t, 1, cfg.Engine.Budget.ResetDay)
+}