test(ui): extend redirect guard meta-test to Tier-2 pattern templates (#204 review)

Fork adversarial review nit: Map_Keys_Should_Not_Begin_With_Forbidden_Prefix
covered only Tier-1 Map keys, not the Tier-2 PatternRule templates. That guard
invariant is the load-bearing protection for the un-prefix-guarded /api|/artwork|
/docs|/openapi surface, so make it self-enforcing over ALL rules — a future
prefix-violating template now fails the test instead of slipping through.

Exposes internal LegacyUiRedirects.PatternTemplates (InternalsVisibleTo already
set for ErsatzTV.Tests); stores the raw template on PatternRule.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

ErsatzTV/LegacyUiRedirects.cs

@@ -280,14 +280,23 @@ public static class LegacyUiRedirects
     private static bool IsPositiveInteger(string segment) =>
         int.TryParse(segment, NumberStyles.None, CultureInfo.InvariantCulture, out int id) && id > 0;
 
+    // Exposed for the guard-invariant meta-test (InternalsVisibleTo ErsatzTV.Tests):
+    // the source-side template of every Tier-2 rule, so the test can assert no
+    // template — not just no Tier-1 Map key — begins with a forbidden prefix.
+    internal static IEnumerable<string> PatternTemplates => Patterns.Select(rule => rule.Template);
+
     private sealed class PatternRule
     {
         public PatternRule(string template, string target)
         {
+            Template = template;
             Segments = template.Split('/', StringSplitOptions.RemoveEmptyEntries);
             Target = target;
         }
 
+        // The raw legacy route template, e.g. "/channels/{id}" (source side).
+        public string Template { get; }
+
         // e.g. ["channels", "{id}"]
         public string[] Segments { get; }