Fix admin crash on fresh installs by normalizing risk details and bump to 1.7.3

[happyloa]

Motivation

• Fresh installs could show a critical error in the admin "Potential Risks" area and hide "Ignored Plugins" and "Notifications" because stored risk payloads contained non-array details.vulnerabilities values.
• Iterating or calling count() on null/unexpected types caused a fatal error that prevented the admin page from rendering fully.
• Make risk loading and admin rendering resilient to legacy or malformed stored data so the admin UI always displays safely.
• Prepare a patch release by bumping the plugin version to 1.7.3.

Description

• Add normalizeDetails() to src/Repository/RiskRepository.php to coerce and sanitize details.vulnerabilities into an array and filter out invalid entries.
• Harden admin logic by introducing getRiskVulnerabilities() in src/AdminPage.php and updating uses so counts and iterations only run on validated arrays.
• Update the admin template templates/admin-page.php to reference the normalized vulnerabilities array and avoid direct access to possibly-null keys.
• Bump plugin metadata and constants: update site-add-on-watchdog.php, src/Version.php, and readme.txt to 1.7.3 and add a changelog entry.

Testing

• No automated tests were executed as part of this change.

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Codex Task