language_model: Remove dependencies on individual model provider crates (#25503)

This PR removes the dependencies on the individual model provider crates
from the `language_model` crate.

The various conversion methods for converting a `LanguageModelRequest`
into its provider-specific request type have been inlined into the
various provider modules in the `language_models` crate.

The model providers we provide via Zed's cloud offering get to stay, for
now.

Release Notes:

- N/A

crates/language_model/src/model/mod.rs

@@ -1,7 +1,3 @@
 pub mod cloud_model;
 
-pub use anthropic::Model as AnthropicModel;
 pub use cloud_model::*;
-pub use lmstudio::Model as LmStudioModel;
-pub use ollama::Model as OllamaModel;
-pub use open_ai::Model as OpenAiModel;