Remove setters from collection properties

[ShivangiReja]

We have removed most of the setters from the collection properties in this PR. This PR removes the remaining setters from the public collection properties.

Note: A setter is only generated for nullable collections. For more details, please see this conversation.

[joseharriaga]

Thank you, Shivangi! After this change:

• AssistantModificationOptions.DefaultTools
• CodeInterpreterToolResources.FileIds
• FileSearchToolResources.VectorStoreIds
• VectorStoreCreationOptions.FileIds

The only collection properties left that have a setter are the following:

• MessageCreationOptions.Attachments
• AssistantCreationOptions.Metadata
• AssistantModificationOptions.Metadata
• MessageCreationOptions.Metadata
• MessageModificationOptions.Metadata
• RunModificationOptions.Metadata
• ThreadCreationOptions.Metadata
• ThreadModificationOptions.Metadata
• VectoreStoreCreationOptions.Metadata
• VectorStoreModificationOptions.Metadata

Paging in @trrwilson because we talked about something related a while ago. The properties above are getting a setter from the generator because they are explicitly specified as nullable in the TypeSpec (which itself comes from the swagger spec). Now, in basically every part of the REST API that I have encountered so far, setting a nullable property to null does not imply anything special and it's equivalent to not setting it at all. Assuming null doesn't mean anything special here either, I wonder if we should add a simple change to customize these properties to remove the setter in favor of consistency and a simpler API.

[joseharriaga]

@ShivangiReja : I have approved this PR, and you can merge it. If we decide to revise these other properties, they can go in a separate PR. 😊