fix(moysklad): trailing slash on BaseUrl so HttpClient keeps /1.2/ in path

Logs showed every outbound MoySklad call was hitting
  https://api.moysklad.ru/api/remap/entity/organization
instead of the intended
  https://api.moysklad.ru/api/remap/1.2/entity/organization

Cause: per RFC 3986 §5.3, when HttpClient resolves a relative URI against
a base URI whose path does not end with '/', the last segment of the base
path is discarded. So BaseAddress "…/api/remap/1.2" + relative "entity/…"
produced "…/api/remap/entity/…". MoySklad returned 503 and we translated
it into a useless "401 сессия истекла" for the user.

Fixes:
- Append trailing slash to BaseUrl.
- Surface the real upstream status + body: MoySkladApiResult<T> wrapper,
  and the controller now maps 401/403 → "invalid token", 502/503 →
  "MoySklad unavailable", anything else → "MoySklad returned {code}: {body}".
  No more lying-as-401.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/food-market.api/Controllers/Admin/MoySkladImportController.cs

@@ -22,12 +22,23 @@ public async Task<IActionResult> TestConnection([FromBody] TestRequest req, Canc
         if (string.IsNullOrWhiteSpace(req.Token))
             return BadRequest(new { error = "Token is required." });
 
-        var org = await _svc.TestConnectionAsync(req.Token, ct);
-        return org is null
-            ? Unauthorized(new { error = "Токен недействителен или нет доступа к API." })
-            : Ok(new { organization = org.Name, inn = org.Inn });
+        var result = await _svc.TestConnectionAsync(req.Token, ct);
+        if (!result.Success)
+        {
+            var msg = result.StatusCode switch
+            {
+                401 or 403 => "Токен недействителен или не имеет доступа к API.",
+                503 or 502 => "МойСклад временно недоступен. Повтори через минуту.",
+                _ => $"МойСклад вернул {result.StatusCode}: {Truncate(result.Error, 400)}",
+            };
+            return StatusCode(result.StatusCode ?? 502, new { error = msg });
+        }
+        return Ok(new { organization = result.Value!.Name, inn = result.Value.Inn });
     }
 
+    private static string? Truncate(string? s, int max)
+        => string.IsNullOrEmpty(s) ? s : (s.Length <= max ? s : s[..max] + "…");
+
     [HttpPost("import-products")]
     public async Task<ActionResult<MoySkladImportResult>> ImportProducts([FromBody] ImportRequest req, CancellationToken ct)
     {

src/food-market.infrastructure/Integrations/MoySklad/MoySkladClient.cs

@@ -4,11 +4,20 @@
 
 namespace foodmarket.Infrastructure.Integrations.MoySklad;
 
+public record MoySkladApiResult<T>(bool Success, T? Value, int? StatusCode, string? Error)
+{
+    public static MoySkladApiResult<T> Ok(T value) => new(true, value, 200, null);
+    public static MoySkladApiResult<T> Fail(int status, string? error) => new(false, default, status, error);
+}
+
 // Thin HTTP wrapper over MoySklad JSON-API 1.2. Caller supplies the personal token per request
 // — we never persist it.
 public class MoySkladClient
 {
-    private const string BaseUrl = "https://api.moysklad.ru/api/remap/1.2";
+    // Trailing slash is critical: otherwise HttpClient drops the last path segment
+    // when resolving relative URIs (RFC 3986 §5.3), so "entity/product" would hit
+    // "/api/remap/entity/product" instead of "/api/remap/1.2/entity/product".
+    private const string BaseUrl = "https://api.moysklad.ru/api/remap/1.2/";
     private static readonly JsonSerializerOptions Json = new(JsonSerializerDefaults.Web);
 
     private readonly HttpClient _http;
@@ -29,13 +38,20 @@ private HttpRequestMessage Build(HttpMethod method, string pathAndQuery, string
         return req;
     }
 
-    public async Task<MsOrganization?> WhoAmIAsync(string token, CancellationToken ct)
+    public async Task<MoySkladApiResult<MsOrganization>> WhoAmIAsync(string token, CancellationToken ct)
     {
         using var req = Build(HttpMethod.Get, "entity/organization?limit=1", token);
         using var res = await _http.SendAsync(req, ct);
-        if (!res.IsSuccessStatusCode) return null;
+        if (!res.IsSuccessStatusCode)
+        {
+            var body = await res.Content.ReadAsStringAsync(ct);
+            return MoySkladApiResult<MsOrganization>.Fail((int)res.StatusCode, body);
+        }
         var list = await res.Content.ReadFromJsonAsync<MsListResponse<MsOrganization>>(Json, ct);
-        return list?.Rows.FirstOrDefault();
+        var org = list?.Rows.FirstOrDefault();
+        return org is null
+            ? MoySkladApiResult<MsOrganization>.Fail(200, "Empty organization list returned by MoySklad.")
+            : MoySkladApiResult<MsOrganization>.Ok(org);
     }
 
     public async IAsyncEnumerable<MsProduct> StreamProductsAsync(

src/food-market.infrastructure/Integrations/MoySklad/MoySkladImportService.cs

@@ -32,10 +32,8 @@ public class MoySkladImportService
         _log = log;
     }
 
-    public async Task<MsOrganization?> TestConnectionAsync(string token, CancellationToken ct)
-    {
-        return await _client.WhoAmIAsync(token, ct);
-    }
+    public Task<MoySkladApiResult<MsOrganization>> TestConnectionAsync(string token, CancellationToken ct)
+        => _client.WhoAmIAsync(token, ct);
 
     public async Task<MoySkladImportResult> ImportProductsAsync(
         string token,