Enforce single PHP instance per CLI worker for correct file locking

[adamziel]

Summary

When PHP acquires file locks via flock(), the locks are managed at the kernel level per-process. Multiple PHP WASM instances within the same OS process would share those locks, defeating the purpose of file locking for concurrent access control.

This PR ensures CLI workers use exactly one PHP instance by:

• Adding maxPhpInstances option to PHPRequestHandler that, when set to 1, bypasses PHPProcessManager entirely and uses a single PHP instance directly
• Refactoring the spawn handler into a unified createPhpSpawnHandler() that accepts a configurable phpCliHandler for subprocess spawning
• CLI workers now use createPhpSpawnHandler without a PHP CLI handler, meaning proc_open('php ...') returns exit code 127 instead of spawning in-process
• Web environments continue using sandboxedSpawnHandlerFactory which spawns PHP instances via PHPProcessManager

New Spawn Handler API

// Core handler - works everywhere
createPhpSpawnHandler({
  getPrimaryPhp: () => Promise<PHP>,  // For ls, pwd commands
  phpCliHandler?: PHPCliHandler,       // Optional: handles php commands
})

// Helper for multi-instance mode (web)
createProcessManagerCliHandler(acquirePHPInstance)

// Legacy wrapper - auto-detects mode based on processManager presence
sandboxedSpawnHandlerFactory(requestHandler)

Test plan

• TypeScript compiles without errors
• Lint passes
• sandboxedSpawnHandlerFactory tests pass
• Manual test: CLI worker handles HTTP requests correctly
• Manual test: proc_open('ls') works in CLI
• Manual test: proc_open('php ...') returns exit code 127 in CLI