feat(api,core,infra): refactor endpoint routing and add institution account discovery

- Move login and refresh endpoints under a unified /auth route group
- Relocate institution connection logic from /accounts to /institutions
- Implement GET /institutions/{id}/available to fetch real-time Plaid
accounts
- Introduce HttpRequestBuilder utility to streamline integration testing
- Enhance PlaidService with GetAccountsAsync to support account fetching
- Clean up global usings and project structure for better domain
isolation

tests/FiscalOS.API.Tests/Assertions/HttpResponseMessageAssertions.cs

@@ -21,12 +21,6 @@ internal sealed class HttpResponseMessageAssertions(
     HttpStatusCode expectedStatusCode
   )
   {
-    _chain.ForCondition(Subject.Content.Headers.ContentType?.MediaType is "application/json")
-      .FailWith(
-        "Expected response to be application/json, but found {0}",
-        Subject.Content.Headers.ContentType?.MediaType
-      );
-
     _chain.ForCondition(Subject.StatusCode == expectedStatusCode)
       .FailWith(
         "Expected response status code to be {0}, but found {1}",
@@ -34,6 +28,13 @@ internal sealed class HttpResponseMessageAssertions(
         Subject.StatusCode
       );
 
+    _chain.ForCondition(Subject.Content.Headers.ContentType?.MediaType is "application/json")
+      .FailWith(
+        "Expected response to be application/json, but found {0}",
+        Subject.Content.Headers.ContentType?.MediaType
+      );
+
+
     var content = await Subject.Content.ReadFromJsonAsync<T>();
 
     _chain.ForCondition(content is not null)