[azure-http-specs] Enhance tests for `@override`

Since we got more use cases regarding `@override`, we should add more tests in spector to ensure the subtle behavior discrepancies introduced by `@override` could be tested in emitter:
1. Optionality change
2. Sequence change
3. Other possible usages

A real use case in key vault about optionality change [here](https://github.com/Azure/azure-rest-api-specs/pull/34032).

Start by locating the spector emitter tests and reviewing how existing @override cases are covered. Use the linked Key Vault example to understand the optionality change, then add coverage for optionality, sequence changes, and other relevant usages. Done means the emitter tests exercise these discrepancies and pass.

Written by the indexing model from the issue text.