Add single-product lookup methods to Columbia API client

Adds GetProductBySkuAsync (GET /products?sku=) and GetProductByIdAsync
(GET /products/{id}) for ad-hoc / on-demand refresh of a single record
without pulling the full catalog. Extracts the shared 429-retry send loop
into SendWithRetryAsync, which now also treats 404 as not-found (null) so
single lookups don't throw on a missing SKU/ID.

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

src/PowderCoating.Application/Interfaces/IColumbiaCoatingsApiClient.cs

@@ -32,4 +32,16 @@ public interface IColumbiaCoatingsApiClient
     /// exception as "incomplete pull" and NOT run discontinuation logic against a partial set.
     /// </summary>
     Task<IReadOnlyList<ColumbiaProduct>> GetAllProductsAsync(CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Fetches a single product by exact SKU (<c>GET /products?sku=...</c>), or null if not found.
+    /// For ad-hoc refresh of one record without pulling the whole catalog.
+    /// </summary>
+    Task<ColumbiaProduct?> GetProductBySkuAsync(string sku, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Fetches a single product by WooCommerce product ID (<c>GET /products/{id}</c>), or null if
+    /// not found. Useful when we already store the catalog product's ID and want to refresh it.
+    /// </summary>
+    Task<ColumbiaProduct?> GetProductByIdAsync(int id, CancellationToken cancellationToken = default);
 }

src/PowderCoating.Infrastructure/Services/ColumbiaCoatingsApiClient.cs

@@ -56,48 +56,51 @@ public class ColumbiaCoatingsApiClient : IColumbiaCoatingsApiClient
     public async Task<ColumbiaProductsResponse> GetProductsPageAsync(
         int page, int perPage, CancellationToken cancellationToken = default)
     {
-        if (!IsConfigured)
-            throw new InvalidOperationException("Columbia Coatings API key is not configured (Columbia:ApiKey).");
-
+        EnsureConfigured();
         perPage = Math.Clamp(perPage, 1, ColumbiaIntegrationConstants.MaxPerPage);
         var url = $"{BaseUrl}{ColumbiaIntegrationConstants.ProductsPath}?page={page}&per_page={perPage}";
 
-        for (var attempt = 1; ; attempt++)
-        {
-            using var request = new HttpRequestMessage(HttpMethod.Get, url);
-            request.Headers.Add("X-API-Key", ApiKey);
+        var json = await SendWithRetryAsync(url, $"page {page}", cancellationToken);
+        if (json == null)
+            return new ColumbiaProductsResponse();
 
-            var client = _httpClientFactory.CreateClient();
-            using var response = await client.SendAsync(request, cancellationToken);
+        return JsonSerializer.Deserialize<ColumbiaProductsResponse>(json, JsonOptions) ?? new ColumbiaProductsResponse();
+    }
 
-            if (response.StatusCode == HttpStatusCode.TooManyRequests)
-            {
-                if (attempt > MaxRetriesPer429)
-                    throw new HttpRequestException(
-                        $"Columbia API still rate-limiting after {MaxRetriesPer429} retries on page {page}.");
+    /// <inheritdoc />
+    public async Task<ColumbiaProduct?> GetProductBySkuAsync(string sku, CancellationToken cancellationToken = default)
+    {
+        EnsureConfigured();
+        if (string.IsNullOrWhiteSpace(sku))
+            return null;
 
-                var delaySeconds = GetRetryAfterSeconds(response) ?? DefaultRetryAfterSeconds;
-                _logger.LogWarning(
-                    "Columbia API returned 429 on page {Page} (attempt {Attempt}); waiting {Delay}s before retry.",
-                    page, attempt, delaySeconds);
-                await Task.Delay(TimeSpan.FromSeconds(delaySeconds), cancellationToken);
-                continue;
-            }
+        var url = $"{BaseUrl}{ColumbiaIntegrationConstants.ProductsPath}?sku={Uri.EscapeDataString(sku)}&per_page=1";
+        var json = await SendWithRetryAsync(url, $"sku {sku}", cancellationToken);
+        if (json == null)
+            return null;
 
-            response.EnsureSuccessStatusCode();
+        var response = JsonSerializer.Deserialize<ColumbiaProductsResponse>(json, JsonOptions);
+        return response?.Items.FirstOrDefault();
+    }
 
-            var json = await response.Content.ReadAsStringAsync(cancellationToken);
-            var result = JsonSerializer.Deserialize<ColumbiaProductsResponse>(json, JsonOptions);
-            return result ?? new ColumbiaProductsResponse();
-        }
+    /// <inheritdoc />
+    public async Task<ColumbiaProduct?> GetProductByIdAsync(int id, CancellationToken cancellationToken = default)
+    {
+        EnsureConfigured();
+        // The by-id endpoint returns a bare product object (not the {items,pagination} envelope).
+        var url = $"{BaseUrl}{ColumbiaIntegrationConstants.ProductsPath}/{id}";
+        var json = await SendWithRetryAsync(url, $"id {id}", cancellationToken);
+        if (json == null)
+            return null;
+
+        return JsonSerializer.Deserialize<ColumbiaProduct>(json, JsonOptions);
     }
 
     /// <inheritdoc />
     public async Task<IReadOnlyList<ColumbiaProduct>> GetAllProductsAsync(
         CancellationToken cancellationToken = default)
     {
-        if (!IsConfigured)
-            throw new InvalidOperationException("Columbia Coatings API key is not configured (Columbia:ApiKey).");
+        EnsureConfigured();
 
         var all = new List<ColumbiaProduct>();
 
@@ -119,6 +122,51 @@ public class ColumbiaCoatingsApiClient : IColumbiaCoatingsApiClient
         return all;
     }
 
+    /// <summary>Throws when no API key is configured so callers fail fast rather than 401.</summary>
+    private void EnsureConfigured()
+    {
+        if (!IsConfigured)
+            throw new InvalidOperationException("Columbia Coatings API key is not configured (Columbia:ApiKey).");
+    }
+
+    /// <summary>
+    /// Issues a GET with the API key header and returns the response body. Retries on HTTP 429
+    /// (honoring Retry-After) up to <see cref="MaxRetriesPer429"/>. Returns null on 404 so
+    /// single-product lookups surface "not found" without throwing; throws on any other non-success.
+    /// <paramref name="describe"/> is a short label (e.g. "page 3", "sku ABC") for log/error context.
+    /// </summary>
+    private async Task<string?> SendWithRetryAsync(string url, string describe, CancellationToken cancellationToken)
+    {
+        for (var attempt = 1; ; attempt++)
+        {
+            using var request = new HttpRequestMessage(HttpMethod.Get, url);
+            request.Headers.Add("X-API-Key", ApiKey);
+
+            var client = _httpClientFactory.CreateClient();
+            using var response = await client.SendAsync(request, cancellationToken);
+
+            if (response.StatusCode == HttpStatusCode.TooManyRequests)
+            {
+                if (attempt > MaxRetriesPer429)
+                    throw new HttpRequestException(
+                        $"Columbia API still rate-limiting after {MaxRetriesPer429} retries ({describe}).");
+
+                var delaySeconds = GetRetryAfterSeconds(response) ?? DefaultRetryAfterSeconds;
+                _logger.LogWarning(
+                    "Columbia API returned 429 ({Describe}, attempt {Attempt}); waiting {Delay}s before retry.",
+                    describe, attempt, delaySeconds);
+                await Task.Delay(TimeSpan.FromSeconds(delaySeconds), cancellationToken);
+                continue;
+            }
+
+            if (response.StatusCode == HttpStatusCode.NotFound)
+                return null;
+
+            response.EnsureSuccessStatusCode();
+            return await response.Content.ReadAsStringAsync(cancellationToken);
+        }
+    }
+
     /// <summary>
     /// Parses the <c>Retry-After</c> header (delta-seconds or HTTP-date form) into whole seconds,
     /// or null when absent/unparseable so the caller can fall back to a default.