fix(e2e): serialize davinci e2e-ci tasks under CI to avoid port 5829 conflict

We added "parallelism": false to the e2e-ci--**/* targetDefault in nx.json to fix the port 5829 conflicts introduced when reuseExistingServer was changed to !process.env.CI. With reuseExistingServer: false in CI, each per-file Playwright task attempts to start its own dev server, so concurrent tasks race for port 5829 and all but the first fail immediately. Setting parallelism: false enforces global serial scheduling of all e2e-ci file tasks, completing the serialization the PR commit intended.