fix(multigpu): keep unset per-service GPU UUIDs empty, not the string null

[patil2001]

Summary

--offline installs append five keys (OFFLINE_MODE, DISABLE_TELEMETRY, DISABLE_UPDATE_CHECK, WEB_SEARCH_ENABLED, LOCAL_RAG_ENABLED) to .env after schema validation has already run, so the first ods config validate afterwards fails with "Unknown keys not defined in schema" against values the installer itself wrote. All five are now declared in .env.schema.json and documented (commented, neutral defaults) in .env.example.

AI Assistance

AI-assisted diff of writer and validator key sets. The author reviewed descriptions and placement and is responsible for the final diff.

Release Lane

• Stable hotfix targeting release/2.6.x
• Mainline change targeting main
• Next-minor work targeting the next feature/minor release
• Not sure; reviewer should help classify

Stable hotfix reason:

Not targeting the stable release branch directly.

Changed Surface

• Docs only
• Tests only
• Dashboard UI
• Installer
• Core runtime
• Dashboard API
• CI workflows

Validation

• Schema re-parsed with json.load - passed; all five keys resolve under properties.
• Keys present in .env.example - verified.