assistant: Overhaul provider infrastructure (#14929)

<img width="624" alt="image"
src="https://github.com/user-attachments/assets/f492b0bd-14c3-49e2-b2ff-dc78e52b0815">

- [x] Correctly set custom model token count
- [x] How to count tokens for Gemini models?
- [x] Feature flag zed.dev provider
- [x] Figure out how to configure custom models
- [ ] Update docs

Release Notes:

- Added support for quickly switching between multiple language model
providers in the assistant panel

---------

Co-authored-by: Antonio <antonio@zed.dev>

crates/language_model/src/language_model.rs

@@ -1,7 +1,84 @@
 mod model;
+pub mod provider;
+mod registry;
 mod request;
 mod role;
+pub mod settings;
+
+use std::sync::Arc;
+
+use anyhow::Result;
+use client::Client;
+use futures::{future::BoxFuture, stream::BoxStream};
+use gpui::{AnyView, AppContext, AsyncAppContext, SharedString, Task, WindowContext};
 
 pub use model::*;
+pub use registry::*;
 pub use request::*;
 pub use role::*;
+
+pub fn init(client: Arc<Client>, cx: &mut AppContext) {
+    settings::init(cx);
+    registry::init(client, cx);
+}
+
+pub trait LanguageModel: Send + Sync {
+    fn id(&self) -> LanguageModelId;
+    fn name(&self) -> LanguageModelName;
+    fn provider_name(&self) -> LanguageModelProviderName;
+    fn telemetry_id(&self) -> String;
+
+    fn max_token_count(&self) -> usize;
+
+    fn count_tokens(
+        &self,
+        request: LanguageModelRequest,
+        cx: &AppContext,
+    ) -> BoxFuture<'static, Result<usize>>;
+
+    fn stream_completion(
+        &self,
+        request: LanguageModelRequest,
+        cx: &AsyncAppContext,
+    ) -> BoxFuture<'static, Result<BoxStream<'static, Result<String>>>>;
+}
+
+pub trait LanguageModelProvider: 'static {
+    fn name(&self) -> LanguageModelProviderName;
+    fn provided_models(&self, cx: &AppContext) -> Vec<Arc<dyn LanguageModel>>;
+    fn is_authenticated(&self, cx: &AppContext) -> bool;
+    fn authenticate(&self, cx: &AppContext) -> Task<Result<()>>;
+    fn authentication_prompt(&self, cx: &mut WindowContext) -> AnyView;
+    fn reset_credentials(&self, cx: &AppContext) -> Task<Result<()>>;
+}
+
+pub trait LanguageModelProviderState: 'static {
+    fn subscribe<T: 'static>(&self, cx: &mut gpui::ModelContext<T>) -> Option<gpui::Subscription>;
+}
+
+#[derive(Clone, Eq, PartialEq, Hash, Debug)]
+pub struct LanguageModelId(pub SharedString);
+
+#[derive(Clone, Eq, PartialEq, Hash, Debug)]
+pub struct LanguageModelName(pub SharedString);
+
+#[derive(Clone, Eq, PartialEq, Hash, Debug)]
+pub struct LanguageModelProviderName(pub SharedString);
+
+impl From<String> for LanguageModelId {
+    fn from(value: String) -> Self {
+        Self(SharedString::from(value))
+    }
+}
+
+impl From<String> for LanguageModelName {
+    fn from(value: String) -> Self {
+        Self(SharedString::from(value))
+    }
+}
+
+impl From<String> for LanguageModelProviderName {
+    fn from(value: String) -> Self {
+        Self(SharedString::from(value))
+    }
+}