test(bff): documents endpoint forwards the base64 file to the domain (refs #103)

RED: the self-service documents endpoint takes the base64 file (+ fileName/contentType)
as JSON — the portal encodes client-side — with the bsn from the token, and forwards
all of it to the domain. IDomainClient.ProvideDocumentsAsync grows accordingly.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

services/bff/Bff.Tests/BffFactory.cs

@@ -94,15 +94,15 @@ internal sealed class FakeDomainClient : IDomainClient
         return Task.FromResult(WithdrawSucceeds);
     }
 
-    public (string RegistrationId, string Bsn)? DocumentsProvidedFor { get; private set; }
+    public (string RegistrationId, string Bsn, string ContentBase64, string? FileName, string? ContentType)? DocumentsProvidedFor { get; private set; }
 
     /// <summary>Whether the fake domain reports the provide-documents as done (true → 204) or
     /// not-found/not-owned (false → 404). Tests set this to exercise the relay.</summary>
     public bool ProvideDocumentsSucceeds { get; set; } = true;
 
-    public Task<bool> ProvideDocumentsAsync(string registrationId, string bsn, CancellationToken ct = default)
+    public Task<bool> ProvideDocumentsAsync(string registrationId, string bsn, string contentBase64, string? fileName, string? contentType, CancellationToken ct = default)
     {
-        DocumentsProvidedFor = (registrationId, bsn);
+        DocumentsProvidedFor = (registrationId, bsn, contentBase64, fileName, contentType);
         return Task.FromResult(ProvideDocumentsSucceeds);
     }