Add flatten property tests for unknown type and read-only properties

[Copilot]

Adds two test cases to packages/azure-http-specs/specs/azure/client-generator-core/flatten-property to validate flatten property behavior in edge cases.

Test 1: Flatten property with unknown type

• Model with properties?: unknown field decorated with @flattenProperty
• Validates flattening behavior when property type is non-model

model FlattenUnknownModel {
  name: string;
  
  #suppress "@azure-tools/typespec-azure-core/no-unknown" "For testing purposes"
  #suppress "@azure-tools/typespec-azure-core/no-legacy-usage" "Testing backcompat"
  @Azure.ClientGenerator.Core.Legacy.flattenProperty
  properties?: unknown;
}

Test 2: Flatten property with all read-only properties

• Model where all flattened properties use @visibility(Lifecycle.Read)
• Tests both required and optional flatten property fields

model SolutionProperties {
  @visibility(Lifecycle.Read)
  solutionId?: string;
  @visibility(Lifecycle.Read)
  title?: string;
  @visibility(Lifecycle.Read)
  content?: string;
}

model Solution {
  name: string;
  
  @Azure.ClientGenerator.Core.Legacy.flattenProperty
  properties: SolutionProperties;
  
  @Azure.ClientGenerator.Core.Legacy.flattenProperty
  propertiesOptional?: SolutionProperties;
}

Both tests include corresponding operations and mock API definitions following existing spec patterns.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Spector] Add more tests for flatten property</issue_title>
<issue_description>Add two tests under packages/azure-http-specs/specs/azure/client-generator-core/flatten-property:

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;

/cc @MaryGao </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [Spector] Add more tests for flatten property #3711

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