refactor(api): introduce DTOs for institutional account discovery

- Replace raw Plaid entity response with a dedicated Response wrapper
- Map Plaid accounts to AccountDto to decouple API from external SDK
models
- Update integration tests to assert against the new response structure
- Add JsonConstructors to DTO records for reliable deserialization

src/FiscalOS.API/Institutions/GetAvailable/Endpoint.cs

@@ -44,7 +44,8 @@ internal static class Endpoint
 
     var accessToken = await encryptor.DecryptAsyncFor(user, plaidMetadata.EncryptedAccessToken, ct);
     var accounts = await plaidService.GetAccountsAsync(accessToken);
+    var accountsDtos = accounts.Select(AccountDto.FromPlaidAccount);
 
-    return Results.Ok(accounts);
+    return Results.Ok(Response.From(accountsDtos));
   }
 }