Disable old example tooling

Part of #4831.

RFC: https://github.com/Azure/azure-rest-api-specs/blob/rfc/unified-examples-format/documentation/rfc/unified-examples-format.md (§9 Phase 3)

**Disable the old Swagger-based example tooling** once every service is migrated (#4839) and all consumers are on the new format (TCGC + consumers #4836).

### Scope
- Turn off the legacy `x-ms-examples` validation/generation paths (e.g. OAV-based example handling superseded by `examples-validate` #4832 and `examples-scaffold` #4837).
- Stop emitting the **transitional `x-ms-examples` JSON** from `typespec-autorest` (#4835); `x-id` becomes the sole linkage.
- New examples are authored only in the new format; CI enforces it.

### Out of scope
- Deleting the legacy `x-ms-examples` JSON files (tracked separately).

Depends on #4839 (all services migrated) and #4836 (consumers on new format).

Start with the RFC at documentation/rfc/unified-examples-format/rfc/unified-examples-format.md, §9 Phase 3, then review the legacy OAV example paths, examples-validate (#4832), examples-scaffold (#4837), and typespec-autorest (#4835). Confirm #4839 and #4836 are complete before changing anything; done means legacy validation and generation are disabled, transitional x-ms-examples output stops, and CI accepts only the new format without deleting legacy JSON files.

Written by the indexing model from the issue text.