Cache JSON schema generated in CompleteAsync<T>

[stephentoub]

Currently each call to CompleteAsync<T> generates a schema:

extensions/src/Libraries/Microsoft.Extensions.AI/ChatCompletion/ChatClientStructuredOutputExtensions.cs

Line 137 in a7e4f3b

var schemaNode = (JsonObject)serializerOptions.GetJsonSchemaAsNode(typeof(T), new()

This should be cached, e.g. via a ConditionalWeakTable associated with the corresponding JsonSerializer.

[eiriktsarpalis]

To be clear, this should be reusing the same caching infrastructure used by the existing InferSchema helpers.

[stephentoub]

To be clear, this should be reusing the same caching infrastructure used by the existing InferSchema helpers.

Only tangentially-related, you thought we should consider exposing some of that publicly, yes? If so, could you open an issue to track? Thanks.

[eiriktsarpalis]

Yes, given the current shape of function calling exposing these helpers to other implementers is almost mandatory.

[stephentoub]

Being addressed in #5513