AI auditor: surface the real connection error instead of swallowing it

The Test-AI button called AuditAsync, which caught every exception and returned
null, and used EnsureSuccessStatusCode() (discarding the response body). So a
failing AI service only ever produced a generic 'no response' message with no
detail — impossible to diagnose.

- Add IAiAuditor.TestAsync: runs the real call and returns a detailed Persian
  diagnostic — HTTP status + response body on non-2xx, raw body when the shape
  isn't OpenAI-compatible, and network/proxy/timeout specifics on exceptions.
- AuditAsync now logs the actual HTTP status + response body (and proxy state)
  instead of a bare warning, so server logs show why a call failed.
- ExtractContent / ParseVerdict no longer throw on unexpected JSON; they return
  null so the caller can show the raw body.
- Settings 'Test AI' button uses TestAsync; result box renders multi-line and
  switches to alert-error styling when the test fails.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/JobsMedical.Web/Pages/Admin/Settings.cshtml

@@ -16,7 +16,11 @@
     @if (Model.DemoMsg is not null) { <div class="alert alert-success">@Model.DemoMsg</div> }
     @if (Model.SmsTest is not null) { <div class="alert alert-success">@Model.SmsTest</div> }
     @if (Model.ProxyTest is not null) { <div class="alert alert-success">@Model.ProxyTest</div> }
-    @if (Model.AiTest is not null) { <div class="alert alert-success">@Model.AiTest</div> }
+    @if (Model.AiTest is not null)
+    {
+        <div class="alert @(Model.AiTest.StartsWith("✅") ? "alert-success" : "alert-error")"
+             style="white-space:pre-wrap; word-break:break-word;">@Model.AiTest</div>
+    }
 
     <form method="post">
         <div class="settings-layout">