Add Spector case for Flatten property

[v-jiaodi]

related pr: Azure/typespec-azure#3729
add case:

1. Flatten property case for an non-model type like unknown, typespec like this:

model XXX {
  #suppress "@azure-tools/typespec-azure-core/no-unknown" "FIXME: Update justification, follow aka.ms/tsp/conversion-fix for details"
  properties?: unknown;
}

@@Azure.ClientGenerator.Core.Legacy.flattenProperty(XXX.properties);

2. Flatten property case for all readOnly, typespec like this:

model SolutionProperties {
  @visibility(Lifecycle.Read)
  solutionId?: string;
  @visibility(Lifecycle.Read)
  title?: string;
  @visibility(Lifecycle.Read)
  content?: string;
}
model Solution{
  @Azure.ClientGenerator.Core.Legacy.flattenProperty
  properties: SolutionProperties;
  @Azure.ClientGenerator.Core.Legacy.flattenProperty
  propertiesOptional?: SolutionProperties;
}
op test(@body body:Solution):void;

[MaryGao]

Should be merged Azure/typespec-azure#3729

[v-jiaodi]

For flatten serializer, shouldn’t return item.

[MaryGao]

why this is for all read-only ones?

do we have a case for not all read-only ones?

[v-jiaodi]

add ut for not all read-only ones

[v-jiaodi]

Readonly property has already been excluded in getRequestModelProperties. https://github.com/Azure/autorest.typescript/blob/main/packages/typespec-ts/src/modular/helpers/operationHelpers.ts#L2145

[v-jiaodi]

This logic is the same as the first one, which means this code path will never be executed.