feat: support if-none-match for the extension list endpoint

[joshiste]

Summary

• Wrap the "/" index handler with exthttp.IfNoneMatchHandler using a startup timestamp as ETag
• Allows clients to skip re-fetching the unchanged extension list via If-None-Match header
• Move index handler registration to after all action/discovery registrations for consistency

Test plan

• Verify extension starts and responds to GET / with an ETag header
• Verify GET / with matching If-None-Match header returns 304 Not Modified

🤖 Generated with Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud