fix(backend): 400 instead of 500 on an unparseable peildatum (RB-16)

BIO-019: GET /stamdata/{table}?peildatum= called DateOnly.Parse
directly, which throws FormatException on anything unparseable — an
unhandled 500 (leaking exception detail in Development) instead of
the 400-with-problem-details every other bad-input check in this file
returns. §3c named backend/Stamdata's 71.7% branch coverage (BL-005)
as the weak spot this bug lived in.

Switched to DateOnly.TryParse; an unparseable value now returns
Results.Problem(detail: ..., statusCode: 400), matching the shape the
upload/change-request endpoints already use. Endpoint doc gained
.ProducesProblem(400), so the OpenAPI doc + generated client were
regenerated and committed in this same diff (RB-09's note records a
prior incident where a response-shape change shipped without this and
the drift went unnoticed).

No FE change needed: libs/beheer's stamdata adapter already funnels
every call through runSubmit, which folds any thrown ApiException
(now including this 400) into a generic Result error — ADR-0001's
"the FE renders the decision" already covers "the server rejected
this input".

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/src/BigRegister.Api/Program.cs

@@ -212,12 +212,24 @@ api.MapGet("/stamdata/{table}", (string table, string? peildatum, HttpContext ct
 {
   var t = StamdataCatalog.Find(table);
   if (t is null) return Results.NotFound();
-  var rows = peildatum is { Length: > 0 } p ? t.RowsOn(DateOnly.Parse(p)) : t.Rows();
+  DateOnly? peildatumWaarde = null;
+  // RB-16/BIO-019: DateOnly.Parse threw FormatException on unparseable input, surfacing as
+  // an unhandled 500 (and, in Development, an exception detail leaked to the caller) — an
+  // admin-gated but still user-supplied string needs the same 400 path every other bad-input
+  // check in this file uses, not a crash.
+  if (peildatum is { Length: > 0 } p)
+  {
+    if (!DateOnly.TryParse(p, out var parsed))
+      return Results.Problem(detail: $"Ongeldige peildatum '{p}'.", statusCode: StatusCodes.Status400BadRequest);
+    peildatumWaarde = parsed;
+  }
+  var rows = peildatumWaarde is { } d ? t.RowsOn(d) : t.Rows();
   return Results.Ok(new StamdataTableDto(t.Id, t.Label, t.Columns.Select(ToColumnDto).ToList(), t.Temporal, rows));
 }))
 .Gate("StamdataAdmin")
 .WithName("stamdataTable")
 .Produces<StamdataTableDto>()
+.ProducesProblem(StatusCodes.Status400BadRequest)
 .ProducesProblem(StatusCodes.Status403Forbidden)
 .Produces(StatusCodes.Status404NotFound);
 

backend/swagger.json

@@ -194,6 +194,16 @@
               }
             }
           },
+          "400": {
+            "description": "Bad Request",
+            "content": {
+              "application/problem+json": {
+                "schema": {
+                  "$ref": "#/components/schemas/ProblemDetails"
+                }
+              }
+            }
+          },
           "403": {
             "description": "Forbidden",
             "content": {

backend/tests/BigRegister.Tests/StamdataEndpointTests.cs

@@ -59,6 +59,16 @@ public class StamdataEndpointTests(TestWebApplicationFactory factory) : IClassFi
     Assert.Empty(table.Rows);
   }
 
+  /// RB-16/BIO-019: DateOnly.Parse used to throw FormatException on unparseable input,
+  /// surfacing as an unhandled 500 instead of the 400-with-problem-details every other
+  /// bad-input check in this endpoint file returns.
+  [Fact]
+  public async Task Unparseable_peildatum_is_400_not_500()
+  {
+    var res = await _client.SendAsync(Req(HttpMethod.Get, "/api/v1/stamdata/professions?peildatum=not-a-date", role: "admin"));
+    Assert.Equal(HttpStatusCode.BadRequest, res.StatusCode);
+  }
+
   [Fact]
   public async Task Unknown_table_is_404()
   {