test(api): cover [JsonProperty] path + fail loudly on rename collision (#198, review)

Adversarial-review follow-ups:

- OpenApiSerializerContractTests: add a fifth case, a fully-populated
  ChannelResponseModel — the only DTO with a [JsonProperty("ffmpegProfile")]
  override, i.e. the attribute path of NewtonsoftSchemaNamingTransformer that
  the existing four cases never exercised.
- NewtonsoftSchemaNamingTransformer: a rename colliding with an existing schema
  key now throws InvalidOperationException (naming the type and keys) instead of
  silently overwriting/dropping a property — the generator must never emit a
  lossy spec.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ErsatzTV/Serialization/NewtonsoftSchemaNamingTransformer.cs

@@ -67,11 +67,18 @@ public static class NewtonsoftSchemaNamingTransformer
             return Task.CompletedTask;
         }
 
-        // Rebuild Properties preserving insertion order.
+        // Rebuild Properties preserving insertion order. A rename colliding with an existing key
+        // would silently drop a property from the spec — fail loudly instead (near-unreachable,
+        // but the generator must never emit a lossy document).
         var renamedProperties = new Dictionary<string, IOpenApiSchema>(properties.Count, StringComparer.Ordinal);
         foreach ((string key, IOpenApiSchema value) in properties)
         {
-            renamedProperties[renames.TryGetValue(key, out string newKey) ? newKey : key] = value;
+            string finalKey = renames.TryGetValue(key, out string newKey) ? newKey : key;
+            if (!renamedProperties.TryAdd(finalKey, value))
+            {
+                throw new InvalidOperationException(
+                    $"OpenAPI schema property rename collision on '{clrType.FullName}': renaming '{key}' to '{finalKey}' would overwrite an existing property.");
+            }
         }
 
         schema.Properties = renamedProperties;