test: configure Playwright tests to always run as nightly builds

- Set IS_NIGHTLY=true for all Playwright test runs
- Update CI workflows to pass IS_NIGHTLY=true to test commands
- Update package.json test:browser scripts to include IS_NIGHTLY=true
- Update dialog test to expect 'oss-nightly' instead of 'oss'
- Also update playwright expectations workflow to use IS_NIGHTLY=true

This ensures consistent test behavior regardless of where tests run,
since Playwright tests now always run with IS_NIGHTLY=true.

.github/workflows/ci-tests-e2e.yaml

@@ -70,6 +70,7 @@ jobs:
         run: pnpm exec playwright test --project=chromium --shard=${{ matrix.shardIndex }}/${{ matrix.shardTotal }} --reporter=blob
         env:
           PLAYWRIGHT_BLOB_OUTPUT_DIR: ./blob-report
+          IS_NIGHTLY: 'true'
 
       - name: Upload blob report
         uses: actions/upload-artifact@v4
@@ -117,6 +118,7 @@ jobs:
         run: pnpm exec playwright test --project=${{ matrix.browser }} --reporter=blob
         env:
           PLAYWRIGHT_BLOB_OUTPUT_DIR: ./blob-report
+          IS_NIGHTLY: 'true'
 
       - name: Generate HTML and JSON reports
         if: always()