[Bug] WooCommerce attachment emails fail on Azure App Service — ABSPATH resolves incorrectly in containerized WordPress (v1.2.1)

## Summary

On Azure App Service containerized WordPress deployments, WooCommerce order
notification emails that include attached invoice PDFs fail silently with the
following error in the Azure Email Logs:

Request body validation error. ContentBytes invalid. Should be base64

Basic emails (contact forms, password resets) send successfully. Only emails
with attachments are affected.

---

## Root Cause

In `admin/mailer/class-azure_app_service_email-controller.php`, the
`generate_request_body()` method resolves attachment paths using `ABSPATH`:

```php
foreach ($filePaths as $path) {
if ($path[0] !== '/') {
$path = ABSPATH . trim($path);
}
if (file_exists($path)) { ... }
}
```

On Azure App Service containerized WordPress, `ABSPATH` resolves to
`/var/www/html` while the actual running WordPress root is
`/var/www/wordpress`. The path resolves incorrectly, `file_exists()` returns
false, the attachment is silently dropped, and the ACS REST API rejects the
malformed request body.

The "ContentBytes invalid" error is a red herring — the real issue is a path
mismatch between ABSPATH and the actual WordPress root inside the container.

---

## Steps to Reproduce

1. Deploy WordPress on Azure App Service (Linux, containerized)
2. Install WooCommerce and the App Service Email plugin v1.2.1
3. Place a test order
4. Observe that the New Order and Processing Order emails never arrive
5. Check Azure Email Logs — error reads "ContentBytes invalid. Should be base64"

---

## Expected Behavior

WooCommerce order notification emails send successfully with attached invoice
PDFs via Azure Communication Services.

---

## Actual Behavior

Emails with attachments fail. Emails without attachments succeed.

---

## Proposed Fix

Replace `ABSPATH` with a path derived from `WP_CONTENT_DIR`, which correctly
resolves to the active WordPress root regardless of container layout. Also
adds an empty path guard and immediate trim to prevent related edge case
failures:

```php
foreach ($filePaths as $path) {
$path = trim($path);
if (empty($path)) { continue; }
if ($path[0] !== '/') {
$wp_root = rtrim(str_replace('wp-content', '', WP_CONTENT_DIR), '/');
$path = $wp_root . '/' . $path;
}
if (file_exists($path)) { ... }
}
```

A community fix including a PowerShell automation script and full root cause
analysis is documented here:
https://github.com/JONeillSr/Repair-AppServiceEmailAttachmentPatch

---

## Environment

| Component | Version |
|---|---|
| App Service Email Plugin | v1.2.1 |
| WordPress | 7.x |
| WooCommerce | 8.x+ |
| Azure App Service | Linux, containerized |
| PHP | 8.x |

---

## Workaround

Apply the patch manually via SSH or using the PowerShell automation script
linked above. The fix is confirmed working in production.