E2E Tests: Intermittent TestTerraformModuleTest failures due to stale RBAC permissions from previous test

## Summary

The daily scheduled E2E pipeline (`🧪 E2E Tests`) has a ~27% failure rate (8 failures in the last 30 runs). The most frequent flaky test is `TestTerraformModuleTest`, which intermittently finds 5 permissions instead of the expected 8.

## Root Cause

The tests share a single service principal per `(os, type)` matrix combination. Each test calls `DetachRolesFromSP` at startup to remove all role assignments, then waits **15 seconds** for Azure RBAC de-propagation before creating a new custom role and starting the MPF iteration loop.

However, 15 seconds is often insufficient for Azure to fully de-propagate the previous test's role assignments. When `TestTerraformModuleTest` starts after `TestTerraformACINoTfvarsFile` (which discovers `resourcegroups/{read,write,delete}`), the SP may still effectively have those permissions cached from the prior test's custom role.

### What happens on a failed run

Traced from runs [#21550858823](https://github.com/Azure/mpf/actions/runs/21550858823) (Jan 31) and [#21840777556](https://github.com/Azure/mpf/actions/runs/21840777556) (Feb 9):

| Iteration | What happened | Permission discovered |
|-----------|--------------|----------------------|
| 0 | `azurerm_resource_group.this: Creation complete after 13s` — **RG created successfully**, then `workspaces/read` denied | `workspaces/read` |
| 1 | RG refreshed OK, `workspaces/write` denied | `workspaces/write` |
| 2 | Deployment succeeds → enters destroy phase → "Authorization Successful" | `workspaces/delete` (from destroy) |

**Result:** 5 permissions found (`workspaces/{read,write,delete}` + `deployments/{read,write}` from initial)

**Expected:** 8 permissions (the above + `resourcegroups/{read,write,delete}`)

The 3 missing `resourcegroups/*` permissions were never surfaced as `AuthorizationFailed` errors because the SP still had them from the previous test's role assignment that hadn't fully de-propagated.

### Key evidence

- The resource group is created by **Terraform** (not by MPF — `autoCreateResourceGroup` is `false` for all Terraform tests)
- The RG creation succeeds without any auth error in iteration 0, proving the SP still has `resourcegroups/write` from the prior test
- The same 3 permissions are missing in every occurrence (Jan 31 and Feb 9 show identical patterns)
- The test runs sequentially (`-p 1 -parallel 1`), so the issue is temporal, not concurrent

## Affected Code

- `pkg/usecase/mpfService.go` line 112: `time.Sleep(15 * time.Second)` — wait after role deletion
- `e2eTests/e2eTerraform_test.go:229`: `assert.Equal(t, 8, len(perms))` — exact count assertion

## Failed Runs

| Date | Run ID | Test | Error |
|------|--------|------|-------|
| Feb 24 | [22370086522](https://github.com/Azure/mpf/actions/runs/22370086522) | `TestARMTemplatMultiResourceTemplateFullDeployment` | 54 perms, expected ≥57 (same root cause) |
| Feb 9 | [21840777556](https://github.com/Azure/mpf/actions/runs/21840777556) | `TestTerraformModuleTest` | 5 perms, expected 8 |
| Jan 31 | [21550858823](https://github.com/Azure/mpf/actions/runs/21550858823) | `TestTerraformModuleTest` | 5 perms, expected 8 |

## Proposed Solutions

1. **Short-term:** Increase the RBAC de-propagation wait from 15s to 45s
2. **Medium-term:** Replace the fixed sleep with an active probe — poll until a test API call returns `403 AuthorizationFailed`, confirming old permissions are fully revoked
3. **Long-term:** Use separate service principals per test function to eliminate cross-test RBAC contamination entirely