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/model/mod.rs

@@ -4,57 +4,3 @@ pub use anthropic::Model as AnthropicModel;
 pub use cloud_model::*;
 pub use ollama::Model as OllamaModel;
 pub use open_ai::Model as OpenAiModel;
-
-use serde::{Deserialize, Serialize};
-
-#[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
-pub enum LanguageModel {
-    Cloud(CloudModel),
-    OpenAi(OpenAiModel),
-    Anthropic(AnthropicModel),
-    Ollama(OllamaModel),
-}
-
-impl Default for LanguageModel {
-    fn default() -> Self {
-        LanguageModel::Cloud(CloudModel::default())
-    }
-}
-
-impl LanguageModel {
-    pub fn telemetry_id(&self) -> String {
-        match self {
-            LanguageModel::OpenAi(model) => format!("openai/{}", model.id()),
-            LanguageModel::Anthropic(model) => format!("anthropic/{}", model.id()),
-            LanguageModel::Cloud(model) => format!("zed.dev/{}", model.id()),
-            LanguageModel::Ollama(model) => format!("ollama/{}", model.id()),
-        }
-    }
-
-    pub fn display_name(&self) -> String {
-        match self {
-            LanguageModel::OpenAi(model) => model.display_name().into(),
-            LanguageModel::Anthropic(model) => model.display_name().into(),
-            LanguageModel::Cloud(model) => model.display_name().into(),
-            LanguageModel::Ollama(model) => model.display_name().into(),
-        }
-    }
-
-    pub fn max_token_count(&self) -> usize {
-        match self {
-            LanguageModel::OpenAi(model) => model.max_token_count(),
-            LanguageModel::Anthropic(model) => model.max_token_count(),
-            LanguageModel::Cloud(model) => model.max_token_count(),
-            LanguageModel::Ollama(model) => model.max_token_count(),
-        }
-    }
-
-    pub fn id(&self) -> &str {
-        match self {
-            LanguageModel::OpenAi(model) => model.id(),
-            LanguageModel::Anthropic(model) => model.id(),
-            LanguageModel::Cloud(model) => model.id(),
-            LanguageModel::Ollama(model) => model.id(),
-        }
-    }
-}