ci(embedded-e2e): build SDK and configure test environment

- Add a build-embedded-sdk step to bashlib.sh and wire it into the
  superset-playwright and superset-e2e workflows so the SDK bundle is
  compiled before Playwright runs.
- Set SUPERSET_FEATURE_EMBEDDED_SUPERSET=true via workflow env so the
  feature flag only affects Playwright jobs. Setting it in the shared
  integration test config breaks unrelated Python tests because the
  security manager's guest-user paths access g.user through paths that
  most tests don't mock.
- Add CORS for localhost:9000 and TALISMAN_ENABLED=False to the
  integration test config. Talisman defaults to X-Frame-Options:
  SAMEORIGIN, which blocks the embedded dashboard from rendering
  inside an iframe hosted on a different port.

.github/workflows/bashlib.sh

@@ -59,6 +59,15 @@ build-assets() {
   say "::endgroup::"
 }
 
+build-embedded-sdk() {
+  cd "$GITHUB_WORKSPACE/superset-embedded-sdk"
+
+  say "::group::Build embedded SDK bundle for E2E tests"
+  npm ci
+  npm run build
+  say "::endgroup::"
+}
+
 build-instrumented-assets() {
   cd "$GITHUB_WORKSPACE/superset-frontend"