Closes https://github.com/zed-industries/zed/issues/63314
Closes https://github.com/zed-industries/zed/issues/42584

Follow-up to https://github.com/zed-industries/zed/pull/62652

Removes a lot of duplicated code in favor of throwing it
into`language_core` as well as removing duplicate struct definitions
that were only different because of entirely unused fields.

Furthermore, improves error-messages so that these untagged enum errors
are no longer a thing in an effort to make this more maintainable in the
future.

Lastly, includes a small but nice improvement changing a `Value` to a
`RawValue` so that we don't have to clone for deserialization in that
specific case.

Release Notes:

- Improved streaming for OpenAI-compatible providers.

---------

Co-authored-by: Anant Goel <anant@zed.dev>