feat(chromium): re-add concurrency support for Chromium (#1467)

* feat: add concurrency support to ProcessSupervisor

- Replace the single-slot mutex channel with a configurable semaphore to
allow multiple concurrent tasks.
- Add drain logic to ensure all active
tasks complete before process restarts.

* feat: add chromium-max-concurrency flag

- Add a --chromium-max-concurrency flag (1-6) to the Chromium module to
control how many conversions run in parallel.
- Update LibreOffice to pass maxConcurrency=1 as LibreOffice only supports
a single concurrent conversion.

* test: add integration tests for concurrent Chromium conversions

- Add concurrent request support to the integration test framework with
new step definitions for sending parallel requests and asserting on all
responses.
- Add a feature file for concurrent HTML to PDF conversions.

test/integration/features/debug.feature

@@ -204,8 +204,9 @@ Feature: /debug
           "chromium-ignore-certificate-errors": "false",
           "chromium-incognito": "false",
           "chromium-max-queue-size": "0",
+          "chromium-max-concurrency": "6",
           "chromium-proxy-server": "",
-          "chromium-restart-after": "10",
+          "chromium-restart-after": "100",
           "chromium-start-timeout": "20s",
           "gotenberg-build-debug-data": "true",
           "gotenberg-graceful-shutdown-duration": "30s",