Modify how arrays are resolved by default

[AlitzelMendez]

issue: #6372

[azure-sdk]

All changed packages have been documented.

• ✅ @typespec/http-server-csharp

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@typespec/http-server-csharp - breaking ✏️

Change in Array Scaffolding from TypeSpec to C#,> ,> The default behavior for scaffolding arrays remains unchanged: arrays will continue to be scaffolded as T[] by default. However, for arrays decorated with the @uniqueItems decorator, they will now be scaffolded as ISet<T>, with HashSet<T> as the default implementation.,> ,> Additionally, a new emitter option, collection-type, has been introduced to provide flexibility in how collections are generated:,> - collection-type:,> - array (default): Generates arrays (T[]).,> - enumerable: Generates IEnumerable<T> for collections, with List<T> used as the default implementation when needed.,> ,> #### Unique Items,> For arrays decorated with the @uniqueItems decorator, they will be scaffolded as ISet<T>, regardless of the collection-type option, with HashSet<T> as the default implementation.,> ,> #### Byte Arrays,> The bytes type will always be treated as an array of bytes (byte[]) in C#, regardless of the collection-type option selected.

[azure-sdk]

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[markcowl]

• We also need to ensure the mocks are generated correctly for these types
• This will at least mean updating the logic in mock generation and in Initialiizer to handle enumerable types with an empty list of the appropriate type
• I think we should pick out some specs to do end-to-end validation against manually : we should be able to compile and scaffold services containing array types in multiple contexts (operation param, operation result, model property)

[markcowl]

A few questions.

My biggest ask is that we have a plan for end-to-end testing

• Adding to the test project for the new constraint attributes
• Doing some manual end-to-end tests of actual specs

[markcowl]

A question:

• Thinking more about this, there is still good reason to use arrays for performance, how difficult would it be to add an option that chose between IEnumerable and array (collection-type = "array" | "enumerable")?
• I think ISet<T> should be the default when @uniqueItems is present, regardless of the above.
• I think we want to use IEnumerable<T> in all contexts (Controllers, interfaces, models) when this is chosen.

[AlitzelMendez]

A question:

• Thinking more about this, there is still good reason to use arrays for performance, how difficult would it be to add an option that chose between IEnumerable and array (collection-type = "array" | "enumerable")?
• I think ISet<T> should be the default when @uniqueItems is present, regardless of the above.
• I think we want to use IEnumerable<T> in all contexts (Controllers, interfaces, models) when this is chosen.

I think it should be quite simple to make this an option but let me work really quick on this, which one do we want to make the default?