Skip enterprise tests for forks since they can't run without secrets (#6888)

# Description of Changes
OSS contributions which trigger the enterprise e2e tests will always
fail due to missing secrets (see
https://github.com/Stirling-Tools/Stirling-PDF/actions/runs/28776342146/job/85337748018?pr=6045).
This PR disables them for OSS PRs.

.github/workflows/build-enterprise.yml

@@ -2,7 +2,7 @@ name: Enterprise E2E (Playwright)
 
 # Enterprise Playwright suite — exercises premium-key gated features (audit,
 # teams, analytics) plus full OAuth + SAML logins via the Keycloak compose
-# stacks under testing/compose. Slow and secret-gated, so it runs in three
+# stacks under testing/compose. Slow and secret-gated, so it runs in four
 # situations:
 #
 #   - PRs that touch proprietary / premium / SSO compose / enterprise tests
@@ -12,8 +12,6 @@ name: Enterprise E2E (Playwright)
 #   - on a nightly cron schedule (catches Keycloak image drift, license
 #     expiry, upstream proprietary changes),
 #   - manual workflow_dispatch.
-#
-# Auto-skipped when secrets.PREMIUM_KEY_ENTERPRISE is missing (forks, dependabot).
 
 on:
   workflow_call:
@@ -52,6 +50,10 @@ jobs:
 
   playwright-e2e-enterprise:
     needs: pick
+    # Skip on fork PRs / untrusted authors: they have no PREMIUM_KEY_ENTERPRISE
+    # (nor DEPOT_TOKEN), so the suite can't boot premium and would fail. See the
+    # header comment. GitHub reports the skipped reusable workflow as success.
+    if: needs.pick.outputs.is_fork != 'true'
     runs-on: ${{ needs.pick.outputs.is_fork == 'true' && 'ubuntu-latest' || format('depot-ubuntu-24.04-{0}', inputs.depot_cores || '8') }}
     timeout-minutes: 45
     env: