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.

Makefile

@@ -31,8 +31,9 @@ API-DOWNLOAD-FROM-FROM-MAX-RETRY=4
 API-DISABLE-DOWNLOAD-FROM=false
 API_DISABLE_HEALTH_CHECK_LOGGING=false
 API_ENABLE_DEBUG_ROUTE=false
-CHROMIUM_RESTART_AFTER=10
+CHROMIUM_RESTART_AFTER=100
 CHROMIUM_MAX_QUEUE_SIZE=0
+CHROMIUM_MAX_CONCURRENCY=6
 CHROMIUM_AUTO_START=false
 CHROMIUM_START_TIMEOUT=20s
 CHROMIUM_ALLOW_INSECURE_LOCALHOST=false
@@ -111,6 +112,7 @@ run: ## Start a Gotenberg container
 	--chromium-restart-after=$(CHROMIUM_RESTART_AFTER) \
 	--chromium-auto-start=$(CHROMIUM_AUTO_START) \
 	--chromium-max-queue-size=$(CHROMIUM_MAX_QUEUE_SIZE) \
+	--chromium-max-concurrency=$(CHROMIUM_MAX_CONCURRENCY) \
 	--chromium-start-timeout=$(CHROMIUM_START_TIMEOUT) \
 	--chromium-allow-insecure-localhost=$(CHROMIUM_ALLOW_INSECURE_LOCALHOST) \
 	--chromium-ignore-certificate-errors=$(CHROMIUM_IGNORE_CERTIFICATE_ERRORS) \
@@ -166,6 +168,7 @@ PLATFORM=
 NO_CONCURRENCY=false
 # Available tags:
 # chromium
+# chromium-concurrent
 # chromium-convert-html
 # chromium-convert-markdown
 # chromium-convert-url