test(sqllab): migrate Cypress SQL Lab tests to Playwright

Replace 3 Cypress spec files with a single Playwright spec covering
10 tests: query execution (3), tab management (4), save/share (2),
and chart creation (1). Add chromium-sqllab project in playwright
config with fullyParallel: false for server-side tab state isolation.

Deleted Cypress files:
- cypress/e2e/sqllab/query.test.ts
- cypress/e2e/sqllab/tabs.test.ts
- cypress/e2e/sqllab/sqllab.helper.js
- cypress/e2e/sqllab/_skip.sourcePanel.index.test.js

Author: sadpandajoe