[bug] Playground Tests panel shows "No tests found" since 0.19.0: requestCollectTests returns an empty tree

./repro.sh 0.18.1-nightly.20260909.a   # returns the tree
./repro.sh 0.19.0                      # returns []

--- baml test --list
    Selected 2 test(s)
  root::adds  (<testset>)
  root::group::inner  (<testset>)
--- baml playground
toolchain 0.18.1-nightly.20260909.a protocol 2
build generation 1 test init functions 1
testCollectionResult [{"$type":"testing.SerializedTest","type":"test","name":"root::adds"},{"$type":"testing.SerializedTest","type":"lazyTestSet","name":"root::group"}]

--- baml test --list
    Selected 2 test(s)
  root::adds  (<testset>)
  root::group::inner  (<testset>)
--- baml playground
toolchain 0.19.0 protocol 2
build generation 1 test init functions 1
testCollectionResult []

#!/usr/bin/env bash
# Usage: ./repro.sh <toolchain-version> [port]
# Creates a two-test project pinned to <toolchain-version>, lists tests with the CLI,
# then starts `baml playground` and asks it for the test tree over /api/ws.
set -euo pipefail
VERSION=$1
PORT=${2:-4291}
HERE=$(cd "$(dirname "$0")" && pwd)
DIR=$(mktemp -d)
trap 'kill "${PG_PID:-}" 2>/dev/null || true; rm -rf "$DIR"' EXIT
mkdir -p "$DIR/baml_src"
printf '[package]\nname = "collect-repro"\n\n[toolchain]\nversion = "%s"\n' "$VERSION" > "$DIR/baml.toml"
cat > "$DIR/baml_src/main.baml" <<'BAML'
function add(a: int, b: int) -> int {
    a + b
}

test "adds" {
    assert.equal(add(1, 2), 3)
}

testset "group" {
    test "inner" {
        assert.equal(add(2, 2), 4)
    }
}
BAML
cd "$DIR"
echo "--- baml test --list"
baml test --list
echo "--- baml playground"
baml playground --port "$PORT" --no-open > "$DIR/playground.log" 2>&1 &
PG_PID=$!
for _ in $(seq 1 60); do
  if (exec 3<>"/dev/tcp/127.0.0.1/$PORT") 2>/dev/null; then break; fi
  sleep 0.5
done
node "$HERE/collect-probe.mjs" "$PORT"

// Usage: node collect-probe.mjs <port>   (Node >= 22 for the global WebSocket)
// Connects to a running `baml playground`, asks for the test tree of every project
// once the build is current, and prints what the server sends back.
const port = process.argv[2] ?? '4265';
const ws = new WebSocket(`ws://127.0.0.1:${port}/api/ws`);
let projects = [];
let asked = false;
ws.onopen = () => ws.send(JSON.stringify({ type: 'requestState' }));
ws.onmessage = (ev) => {
  const m = JSON.parse(ev.data);
  if (m.type === 'hello') console.log('toolchain', m.toolchainVersion, 'protocol', m.playgroundProtocol);
  const n = m.notification;
  if (!n) return;
  if (n.type === 'listProjects') projects = n.projects.map((p) => p.path);
  if (n.type === 'updateProject' && n.update.isBexCurrent && !asked) {
    asked = true;
    const init = n.update.functions.filter((f) => f.name.startsWith('$init_test')).map((f) => f.name);
    console.log('build generation', n.update.generation, 'test init functions', init.length);
    for (const p of projects) ws.send(JSON.stringify({ type: 'requestCollectTests', project: p }));
  }
  if (n.type === 'testCollectionResult') {
    const tree = new TextDecoder().decode(new Uint8Array(n.data ?? []));
    console.log('testCollectionResult', tree, n.expandError ? `expandError=${JSON.stringify(n.expandError)}` : '');
    ws.close();
    process.exit(0);
  }
};
setTimeout(() => { console.log('timeout: no testCollectionResult'); process.exit(2); }, 60000);