specs, bug fix on flatten-property mockapi

[weidongxu-microsoft]

Cadl Ranch Contribution Checklist:

• [ ] I have written a scenario spec
• [ ] I have meaningful @scenario names. Someone can look at the list of scenarios and understand what I'm covering.
• [ ] I have written a mock API
• [ ] I have used @scenarioDocs for extra scenario description and to tell people how to pass my mock api check.

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: f209e6cb67cc49ddfd34d0644bddb6a7bd94e702

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

| Name | Type | | ----------------------------- | ----- | | @azure-tools/cadl-ranch-specs | Patch |

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