Pilot migration (EventGrid) + CI round-trip

[timotheeguerin]

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 (x-id emission: typespec-autorest + transitional JSON #4835) and resolve per version (examples-resolve tool #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.