Add service tier support to Chat and Responses APIs

[ShivangiReja]

Introduces service-tier support for Chat and Responses.

Chat:

• Added extensible enum ChatServiceTier.
• Added ServiceTier property to ChatCompletionOptions (request).
• Added ServiceTier to ChatCompletion and StreamingChatCompletionUpdate (response).

Responses:

• Added extensible enum ResponseServiceTier.
• Added ServiceTier property to ResponseCreationOptions(request).
• Added ServiceTier to OpenAIResponse(response) .

Issue: #592

[christothes]

Just curious - why did we have to do this versus only renaming InternalServiceTier and making it public?

[christothes]

Thinking about it some more, this is so that we can have the differentiated type name for Chat and Responses. 😄

[ShivangiReja]

In the spec, ServiceTier is defined as a common type shared across multiple subclients (like Chat and Responses) but in the .NET SDK, we don’t share types between subclients. Instead, for common types we generate custom wrappers per subclient. That’s why we introduce subclient-specific types, see spec example here:

union DotNetChatServiceTier {
  ServiceTier
}

union DotNetResponseServiceTier {
  ServiceTier
}

This way each subclient gets its own strongly-typed version while still mapping back to the same spec definition.