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

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

crates/zed/Cargo.toml

@@ -56,6 +56,7 @@ install_cli.workspace = true
 isahc.workspace = true
 journal.workspace = true
 language.workspace = true
+language_model.workspace = true
 language_selector.workspace = true
 language_tools.workspace = true
 languages.workspace = true