fix(ci): Remove hardcoded default from Playwright Dockerfile

Remove the default value for PLAYWRIGHT_VERSION ARG so the build
fails if the arg isn't passed. The ensure-playwright-image action
reads the version from browser-integration-tests/package.json and
passes it as a build arg — that is the single source of truth.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: mydea

.github/docker/playwright.Dockerfile

@@ -1,4 +1,6 @@
-ARG PLAYWRIGHT_VERSION=1.56.1
+# PLAYWRIGHT_VERSION is passed as a build arg by the ensure-playwright-image action.
+# The canonical source is dev-packages/browser-integration-tests/package.json.
+ARG PLAYWRIGHT_VERSION
 FROM mcr.microsoft.com/playwright:v${PLAYWRIGHT_VERSION}-noble
 
 # Install yarn (v1) for the monorepo