Refactor WebSocket to Task-based API and update tests

Rewrite WebSocket implementation to a Task-based API: createWebSocket now returns a Task<WebSocket>, uses Task/Schedule primitives (retry, callback, exponential backoff, jitter, maxDelay), adds shouldRetryOnClose, improves binary handling and docs, and removes legacy retry/old task code. Update tests to use an in-process test WebSocket server (_globalSetup.ts) and the Task runner (createRunner), modernize assertions, and cover various close/terminate/retry scenarios. Update vitest config to exclude additional patterns, add Playwright browser commands (startWsServer/stopWsServer) for browser tests, and tweak parallelism/snapshot settings.

Author: steida