This adds support for identifying which worker is running a test file when tests execute concurrently, similar to JEST_WORKER_ID in Jest, VITEST_POOL_ID in Vitest, and MOCHA_WORKER_ID in Mocha.

When running with --test-isolation=process (default), each test file runs in a separate child process and receives a unique worker ID from 1 to N. When running with --test-isolation=none, all tests run in the same process and the worker ID is always 1.

This enables users to allocate separate resources (databases, ports, etc.) for each test worker to avoid conflicts during concurrent execution.

Changes:

• Add WorkerIdPool class to manage worker ID allocation and reuse
• Set NODE_TEST_WORKER_ID environment variable for child processes
• Add context.workerId getter to TestContext class
• Add tests for worker ID functionality
• Add documentation for NODE_TEST_WORKER_ID and context.workerId

Fixes: #55842