[CLI] Spawn OS processes for PHP subprocesses in Blueprint V2 worker

[adamziel]

Motivation

This is the V2 counterpart to PR #3001. When PHP calls proc_open() to spawn a subprocess (like WP-CLI calling PHP), we need to ensure those subprocesses don't interfere with the parent's file locks.

Previously in the V2 worker, sandboxedSpawnHandlerFactory was passed directly without a callback to create new OS processes. Subprocesses ran as new PHP instances within the same Node.js worker, which meant OS-level file locks (via fcntl() and LockFileEx()) could overlap between parent and child.

This PR applies the same pattern from the V1 worker: spawning each subprocess in a separate Node.js worker thread running worker-thread-v2.ts.

Implementation

The bootRequestHandler method now wraps sandboxedSpawnHandlerFactory with a callback that:

1. Spawns a new Node.js worker thread via spawnWorkerThread('v2')
2. Boots a fresh PHP runtime in that worker using bootWorker()
3. Connects the subprocess I/O streams to the parent
4. Terminates the worker when the subprocess exits

Testing

This change affects the experimental Blueprint V2 runner. Run WP-CLI commands that spawn subprocesses:

npx wp-playground-cli server --experimental-blueprints-v2-runner --mount ./my-plugin:/wordpress/wp-content/plugins/my-plugin
# In another terminal:
curl "http://127.0.0.1:9400/?cli=wp+plugin+list"