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/Cargo.toml

@@ -22,12 +22,27 @@ test-support = [
 
 [dependencies]
 anthropic = { workspace = true, features = ["schemars"] }
+anyhow.workspace = true
+client.workspace = true
+collections.workspace = true
+editor.workspace = true
+feature_flags.workspace = true
+futures.workspace = true
+gpui.workspace = true
+http.workspace = true
+menu.workspace = true
 ollama = { workspace = true, features = ["schemars"] }
 open_ai = { workspace = true, features = ["schemars"] }
+proto = { workspace = true, features = ["test-support"] }
 schemars.workspace = true
 serde.workspace = true
+serde_json.workspace = true
+settings.workspace = true
 strum.workspace = true
-proto = { workspace = true, features = ["test-support"] }
+theme.workspace = true
+tiktoken-rs.workspace = true
+ui.workspace = true
+util.workspace = true
 
 [dev-dependencies]
 ctor.workspace = true