refactor(server-manager): extract server helpers and modularize PHP stack manager

[fordimalanda]

Summary

This PR refactors PHPStackManager to reduce complexity and improve maintainability within src/serverManager.ts. Responsibilities such as logging, router generation, and workspace file visibility management have been decoupled and extracted into dedicated helper modules under src/serverHelpers/.

Key Changes

• Extracted ServerLogger (src/serverHelpers/serverLogger.ts):
  • Isolated OutputChannel logging, request counting, and HTTP status code formatting.
• Extracted RouterBuilder (src/serverHelpers/routerBuilder.ts):
  • Moved buildRouterContent and getRouterFilePath logic out of serverManager.ts to streamline router script template handling.
• Extracted FileVisibility Helper (src/serverHelpers/fileVisibility.ts):
  • Created a standalone helper to manage VS Code workspace file exclusions (files.exclude).
• Streamlined PHPStackManager (src/serverManager.ts):
  • Refactored PHPStackManager to focus strictly on process lifecycle management (start, stop, restartServer).
  • Converted _cleanup() and stop() methods to async to guarantee clean resolution of file visibility changes prior to resource teardown.

Verification

• Tested PHP development server startup, routing, and termination to confirm feature parity.
• Verified that HTTP status log formatting and request counting function as expected through ServerLogger.
• Confirmed that toggling workspace file visibility resolves asynchronously without race conditions on process exit.