fix(notify): normalize legacy channel config and restore health diagnostics endpoint

- Add legacy channel config normalization for unmapped smtpHost, webhookUrl,
  channel fields into canonical NotifyChannelConfig
- Restore GET /channels/{channelId}/health endpoint
- Add JsonConverter attribute to ChannelHealthStatus enum
- Add test coverage for legacy row shapes and health contract
- Remove hosted services from test override to isolate channel tests

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/Notify/StellaOps.Notify.WebService/Program.cs

@@ -658,6 +658,43 @@ static void ConfigureEndpoints(WebApplication app)
         .RequireAuthorization(NotifyPolicies.Operator)
         .RequireRateLimiting(NotifyRateLimitPolicies.TestSend);
 
+    apiGroup.MapGet("/channels/{channelId}/health", async (
+            string channelId,
+            IChannelRepository repository,
+            INotifyChannelHealthService healthService,
+            HttpContext context,
+            CancellationToken cancellationToken) =>
+        {
+            if (!TryResolveTenant(context, tenantHeader, out var tenant, out var error))
+            {
+                return error!;
+            }
+
+            if (!TryParseGuid(channelId, out var channelGuid))
+            {
+                return Results.BadRequest(new { error = _t("notify.error.channel_id_must_be_guid") });
+            }
+
+            var channelEntity = await repository.GetByIdAsync(tenant, channelGuid, cancellationToken)
+                .ConfigureAwait(false);
+            if (channelEntity is null)
+            {
+                return Results.NotFound();
+            }
+
+            var channel = ToNotifyChannel(channelEntity);
+            var response = await healthService.CheckAsync(
+                tenant,
+                channel,
+                context.TraceIdentifier,
+                cancellationToken).ConfigureAwait(false);
+
+            return Results.Ok(response);
+        })
+        .WithName("NotifyGetChannelHealth")
+        .WithDescription("Returns connector diagnostics for the specified channel.")
+        .RequireAuthorization(NotifyPolicies.Viewer);
+
     apiGroup.MapDelete("/channels/{channelId}", async (string channelId, IChannelRepository repository, HttpContext context, CancellationToken cancellationToken) =>
         {
             if (!TryResolveTenant(context, tenantHeader, out var tenant, out var error))
@@ -1267,23 +1304,24 @@ static ChannelEntity ToChannelEntity(NotifyChannel channel)
 
 static NotifyChannel ToNotifyChannel(ChannelEntity entity)
 {
-    var config = string.IsNullOrWhiteSpace(entity.Config)
-        ? NotifyChannelConfig.Create("inline-default")
-        : NotifyCanonicalJsonSerializer.Deserialize<NotifyChannelConfig>(entity.Config);
-
     var metadataModel = TryDeserialize<NotifyChannel>(entity.Metadata);
+    if (metadataModel is not null)
+    {
+        return metadataModel;
+    }
 
-    return metadataModel ??
-        NotifyChannel.Create(
-            entity.Id.ToString(),
-            entity.TenantId,
-            entity.Name,
-            ToModelChannelType(entity.ChannelType),
-            config,
-            enabled: entity.Enabled,
-            createdBy: entity.CreatedBy,
-            createdAt: entity.CreatedAt,
-            updatedAt: entity.UpdatedAt);
+    var config = ToNotifyChannelConfig(entity);
+
+    return NotifyChannel.Create(
+        entity.Id.ToString(),
+        entity.TenantId,
+        entity.Name,
+        ToModelChannelType(entity.ChannelType),
+        config,
+        enabled: entity.Enabled,
+        createdBy: entity.CreatedBy,
+        createdAt: entity.CreatedAt,
+        updatedAt: entity.UpdatedAt);
 }
 
 static TemplateEntity ToTemplateEntity(NotifyTemplate template)
@@ -1497,6 +1535,108 @@ static T? TryDeserialize<T>(string? json)
     }
 }
 
+static NotifyChannelConfig ToNotifyChannelConfig(ChannelEntity entity)
+{
+    if (string.IsNullOrWhiteSpace(entity.Config))
+    {
+        return NotifyChannelConfig.Create("inline-default");
+    }
+
+    var config = TryDeserialize<NotifyChannelConfig>(entity.Config);
+    if (config is not null)
+    {
+        return config;
+    }
+
+    try
+    {
+        using var document = JsonDocument.Parse(entity.Config);
+        if (document.RootElement.ValueKind != JsonValueKind.Object)
+        {
+            return NotifyChannelConfig.Create(BuildLegacyChannelSecretRef(entity.Id));
+        }
+
+        var root = document.RootElement;
+        var secretRef = GetOptionalString(root, "secretRef");
+        var target = GetOptionalString(root, "target")
+            ?? GetOptionalString(root, "channel")
+            ?? GetOptionalString(root, "recipient");
+        var endpoint = GetOptionalString(root, "endpoint")
+            ?? GetOptionalString(root, "webhookUrl")
+            ?? GetOptionalString(root, "url");
+        var properties = ReadLegacyChannelProperties(root);
+
+        return NotifyChannelConfig.Create(
+            string.IsNullOrWhiteSpace(secretRef) ? BuildLegacyChannelSecretRef(entity.Id) : secretRef,
+            target,
+            endpoint,
+            properties.Count == 0 ? null : properties);
+    }
+    catch
+    {
+        return NotifyChannelConfig.Create(BuildLegacyChannelSecretRef(entity.Id));
+    }
+}
+
+static string BuildLegacyChannelSecretRef(Guid channelId)
+    => $"legacy://notify/channels/{channelId:N}";
+
+static string? GetOptionalString(JsonElement element, string propertyName)
+{
+    if (!element.TryGetProperty(propertyName, out var property))
+    {
+        return null;
+    }
+
+    return property.ValueKind switch
+    {
+        JsonValueKind.Null or JsonValueKind.Undefined => null,
+        JsonValueKind.String => property.GetString(),
+        _ => property.GetRawText()
+    };
+}
+
+static IReadOnlyList<KeyValuePair<string, string>> ReadLegacyChannelProperties(JsonElement root)
+{
+    var properties = new Dictionary<string, string>(StringComparer.Ordinal);
+
+    if (root.TryGetProperty("properties", out var embeddedProperties) && embeddedProperties.ValueKind == JsonValueKind.Object)
+    {
+        foreach (var property in embeddedProperties.EnumerateObject())
+        {
+            properties[property.Name] = GetJsonPropertyValue(property.Value);
+        }
+    }
+
+    foreach (var property in root.EnumerateObject())
+    {
+        if (IsCanonicalChannelConfigField(property.Name))
+        {
+            continue;
+        }
+
+        properties[property.Name] = GetJsonPropertyValue(property.Value);
+    }
+
+    return properties.ToArray();
+}
+
+static bool IsCanonicalChannelConfigField(string propertyName)
+    => propertyName.Equals("secretRef", StringComparison.OrdinalIgnoreCase)
+        || propertyName.Equals("target", StringComparison.OrdinalIgnoreCase)
+        || propertyName.Equals("endpoint", StringComparison.OrdinalIgnoreCase)
+        || propertyName.Equals("properties", StringComparison.OrdinalIgnoreCase)
+        || propertyName.Equals("limits", StringComparison.OrdinalIgnoreCase)
+        || propertyName.Equals("channel", StringComparison.OrdinalIgnoreCase)
+        || propertyName.Equals("recipient", StringComparison.OrdinalIgnoreCase)
+        || propertyName.Equals("webhookUrl", StringComparison.OrdinalIgnoreCase)
+        || propertyName.Equals("url", StringComparison.OrdinalIgnoreCase);
+
+static string GetJsonPropertyValue(JsonElement element)
+    => element.ValueKind == JsonValueKind.String
+        ? element.GetString() ?? string.Empty
+        : element.GetRawText();
+
 static bool TryResolveTenant(HttpContext context, string tenantHeader, out string tenant, out IResult? error)
 {
     // Delegate to unified StellaOps tenant resolver (claims + canonical headers + legacy headers)