Add admin notification for Varnish purge object count

[Copilot]

Implements admin notification displaying the number of objects purged from Varnish cache. Previously, purge operations provided no visibility into how many objects were invalidated.

Changes

New notification class (Model/PurgeStatistics/Notification.php)

• Implements MessageInterface with NOTICE severity
• Displays: "Last cache purge: X object(s) were purged from Varnish. (Purged at: timestamp)"
• Caches flag data to avoid duplicate DB queries

Enhanced purge handling (Model/PurgeCache.php)

• Captures and parses Varnish HTTP responses containing {"invalidated": <count>}
• Sums counts across all servers (previously discarded)
• Stores statistics via FlagManager with timestamp
• Handles both \r\n and \n line endings

Configuration (etc/adminhtml/di.xml)

• Registers notification as standalone message, separate from critical Varnish warnings

Example

When Varnish purges 2 objects using xkey:

// Varnish VCL returns via vcl_synth:
set req.http.n-gone = xkey.purge("cat_p_694");
return (synth(200, req.http.n-gone));  // Response: {"invalidated": 2}

// PurgeCache now captures this:
$purgedCount = $this->parseVarnishResponse($response);  // 2
$this->storePurgeStatistics($totalObjectsPurged);       // Stores for notification

The notification persists in admin until the next purge operation updates it.

Original prompt

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This section details on the original issue you should resolve

<issue_title>vcl_recv Add admin notification in Magento for number of purged objects</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

@hpowernl For tomorrow only the admin part.

• Fixes vcl_recv Add admin notification in Magento for number of purged objects #11

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