Clarify ARM operation-status guidance for brownfield contracts and non-drop-in status models

[Copilot]

The issue was caused by docs implying ArmOperationStatus as blanket current guidance and a universal replacement for existing custom operation-status models. This update narrows the guidance to existing ARM wire-contract compatibility and clarifies where literal replacement is unsafe.

• Scope correction: operation-status templates are contract-preservation guidance

  • Added an explicit note that operationStatuses / Azure-AsyncOperation patterns describe an existing ARM wire contract.
  • Clarified these templates should be used when preserving brownfield behavior, not as default direction for every new ARM API.

• Drop-in caveat for converted custom models

  • Added a warning that ArmOperationStatus<Properties> is not a literal drop-in when legacy models carry extra top-level fields (e.g., operations, resourceId) or different wire semantics.
  • Directed authors to keep or define explicit status models when the existing contract diverges.

> **Note:** The `operationStatuses` and `Azure-AsyncOperation` patterns document an existing ARM wire
> contract ... Do not read this section as blanket guidance for every new ARM API design.

> **Warning:** `ArmOperationStatus<Properties>` is not a literal drop-in for every converted custom
> model ... keep or author an explicit status model that matches the existing contract.

• Fixes https://github.com/Azure/azure-sdk-pr/issues/2885

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