Update sandbox sample Handlebars bundle

[KirtiRamchandani]

Fixes #1615

Summary

• Replace the sandbox sample's vendored Handlebars 1.0.0 beta bundle with Handlebars 4.7.9.
• Update the sample HTML to load the new bundle and refresh the vendored Handlebars license notice.
• Ignore the vendored handlebars-*.js bundle in ESLint so repo lint checks the sample code rather than third-party distribution output.

Notes

The issue also discussed narrowing postMessage("*"), but the maintainer noted that wildcard targeting is needed for this extension-origin communication path. This PR leaves that messaging behavior unchanged and keeps the scope to the approved Handlebars update.

Tests

• npm install --ignore-scripts
• npm run lint
• npx eslint api-samples/sandbox/sandbox/mainpage.js api-samples/sandbox/sandbox/service-worker.js
• npx prettier --check api-samples/sandbox/sandbox/sandbox.html api-samples/sandbox/sandbox/manifest.json api-samples/sandbox/sandbox/sandbox.md eslint.config.js
• Node smoke test loading api-samples/sandbox/sandbox/handlebars-4.7.9.js and rendering both templates from sandbox.html
• Verified the vendored bundle came from handlebars@4.7.9 dist/handlebars.js, with only trailing whitespace removed for git diff --check
• git diff --check