test(BUG-2684): pin that the copy preflight labels a dropped-then-defaulted key from=default, the case the bug was filed for (#1533)

The labelling fix shipped in TASK-2878 (#1246, codex round 9); existing pins covered only null-source and destination-only defaults.

Claude-Session: https://claude.ai/code/session_01A3rXV7GDLAhBe9mL4V6z2m

internal/server/bug2684_preflight_origin_test.go

@@ -0,0 +1,37 @@
+package server
+
+import "testing"
+
+// BUG-2684, the case it was filed for: the SOURCE declares a key, migration
+// DROPS its value (it cannot convert), and the destination's default fills the
+// key back in. The carried row must say from="default" — the destination chose
+// that value — not "migrated", which would tell the user a value came across
+// that did not survive.
+//
+// Fixed by TASK-2878 (#1246, codex round 9), which labels from the source
+// values that SURVIVED migration (store.CarriedSourceValues) rather than from
+// the source map. The existing pins cover a null source value and a
+// destination-only default; neither is this shape, where the source value was
+// present, non-null and discarded.
+func TestBUG2684_PreflightLabelsADroppedThenDefaultedKeyAsDefault(t *testing.T) {
+	f := newCopyPreflightFixture(t)
+	// The fixture's source has count=7 (number); here the destination's
+	// `count` is a select that cannot hold 7, with a default.
+	dst := mustSchemaCollection(t, f.srv, f.wsB.ID, "Counted B", `{"fields":[
+		{"key":"status","label":"Status","type":"select","options":["open","done"]},
+		{"key":"count","label":"Count","type":"select","options":["x","y"],"default":"x"}
+	]}`)
+	body := f.baseBody()
+	body["target_collection"] = dst.Slug
+	resp := f.ok(body)
+
+	got := carriedByKey(t, resp, "count")
+	if got.Value != "x" || got.From != "default" {
+		t.Fatalf("count: the source's 7 was discarded and the destination default filled it; want value=x from=default, got %+v", got)
+	}
+	// CONTROL: a key whose source value DID survive is still "migrated", so
+	// the leg above is not satisfied by labelling everything "default".
+	if st := carriedByKey(t, resp, "status"); st.From != "migrated" {
+		t.Fatalf("status carried across unchanged and must be from=migrated, got %+v", st)
+	}
+}