test(bff): endpoints, JWT auth and public-safe projection (refs #8)

Failing tests for the BFF walking-skeleton endpoints:
- POST /self-service/registrations rejects missing/malformed/wrong-key/expired
  tokens (401) and, with a valid digid token, forwards the bsn to the domain and
  returns 202 (WebApplicationFactory + a local test signing key, ADR-0010).
- GET /openbaar/register serves public-safe rows anonymously (never the bsn) and
  filters by q.
- OpenbaarProjection.PublicView (pure) filters by id and maps to id+status only.

Endpoints and PublicView are stubs so the tests compile and fail on their
assertions; the green commit implements them.

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

services/bff/Bff.Tests/OpenbaarProjectionTests.cs

@@ -0,0 +1,48 @@
+using Bff.Api;
+
+namespace Bff.Tests;
+
+public class OpenbaarProjectionTests
+{
+    private static readonly ProjectionEntry[] Sample =
+    [
+        new("abc-111", "INGEDIEND", "123456782", "Jan"),
+        new("def-222", "INGEDIEND", "987654321", "Piet"),
+    ];
+
+    [Fact]
+    public void Maps_every_row_to_public_safe_fields_when_no_query()
+    {
+        var view = OpenbaarProjection.PublicView(Sample, null);
+
+        Assert.Equal(2, view.Count);
+        Assert.Equal("abc-111", view[0].Id);
+        Assert.Equal("INGEDIEND", view[0].Status);
+    }
+
+    [Fact]
+    public void Public_view_exposes_only_id_and_status()
+    {
+        // OpenbaarEntry structurally carries only Id + Status — bsn/naam can never leak.
+        var props = typeof(OpenbaarEntry).GetProperties().Select(p => p.Name).ToArray();
+        Assert.Equal(["Id", "Status"], props);
+    }
+
+    [Theory]
+    [InlineData("abc", 1)]
+    [InlineData("ABC", 1)]
+    [InlineData("2", 1)]
+    [InlineData("zzz", 0)]
+    public void Filters_by_id_containing_the_query_case_insensitively(string q, int expected)
+    {
+        var view = OpenbaarProjection.PublicView(Sample, q);
+
+        Assert.Equal(expected, view.Count);
+    }
+
+    [Fact]
+    public void Blank_query_is_treated_as_no_filter()
+    {
+        Assert.Equal(2, OpenbaarProjection.PublicView(Sample, "   ").Count);
+    }
+}