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/Response.cs

@@ -0,0 +1,39 @@
+namespace FiscalOS.API.Institutions.GetAvailable;
+
+internal sealed record Response
+{
+  public IEnumerable<AccountDto> Accounts { get; init; } = [];
+
+  [JsonConstructor]
+  private Response()
+  {
+  }
+
+  public static Response From(IEnumerable<AccountDto> accounts)
+  {
+    return new Response
+    {
+      Accounts = accounts
+    };
+  }
+}
+
+internal sealed record AccountDto
+{
+  public string ProviderId { get; init; } = string.Empty;
+  public string ProviderName { get; init; } = string.Empty;
+
+  [JsonConstructor]
+  private AccountDto()
+  {
+  }
+
+  public static AccountDto FromPlaidAccount(Going.Plaid.Entity.Account plaidAccount)
+  {
+    return new AccountDto
+    {
+      ProviderId = plaidAccount.AccountId,
+      ProviderName = plaidAccount.Name
+    };
+  }
+}