Chrome can remain running after the exporter process is forcibly stopped #687

Expected behaviour

Applications that own the browser should be able to make it terminate with the Node process, including when a forced termination prevents graceful cleanup callbacks from running.

Actual behaviour

On Windows, forcibly stopping a Node process that initialized highcharts-export-server@6.0.0 can leave chrome-headless-shell.exe running after Node has exited. The leftover browser consumes resources and may keep its user data directory in use, interfering with a later exporter launch using the same directory.

The export server has no normal puppeteer.pipe setting. Extra Puppeteer launch options are currently forwarded through the debug configuration only when debug mode is enabled. Using that path requires changing browser debug mode, headless settings, and the debugging port.

Reproduction steps

1. Start a Node application that calls exporter.initExport() and wait for the browser to start.
2. Forcibly terminate that Node process, for example with Stop-Process -Id <pid> -Force in PowerShell.
3. Check whether its chrome-headless-shell.exe process remains running.

In our local comparison, Chrome survived with WebSocket transport and exited with its Node owner when Puppeteer's pipe transport was enabled. The existing --pipe entry in puppeteer.args does not select Puppeteer's pipe transport.

Proposed resolution

Expose an opt-in puppeteer.pipe option, validate an environment variable such as PUPPETEER_PIPE, and pass the value to puppeteer.launch(). Keeping the default false preserves existing behavior for consumers that need WebSocket transport and its debugging endpoint.

Proposed fix: #688.