Pilot migration (EventGrid) + CI round-trip

Part of #4831.

RFC: https://github.com/Azure/azure-rest-api-specs/blob/rfc/unified-examples-format/documentation/rfc/unified-examples-format.md

**Pilot the full flow on EventGrid** before broad rollout.

### Scope
- Migrate EventGrid's `x-ms-examples` to a single `examples.yaml` using `examples-migrate`.
- Validate with `examples-validate`; emit `x-id` (#4835) and resolve per version (#4834).
- Wire **CI round-trip validation**: materialized examples for each version match the original `x-ms-examples` (for migrated versions).

Depends on #4832, #4833, #4834, #4835.

Read the unified examples RFC and the prerequisite issues #4832, #4833, #4834, and #4835 first. Use EventGrid as the pilot with examples-migrate and examples-validate, then trace the CI workflow that can compare materialized examples for each version with the original x-ms-examples. Done means the migration validates, x-id and per-version resolution work, and the CI round-trip passes.

Written by the indexing model from the issue text.