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/collab/Cargo.toml

@@ -90,6 +90,7 @@ git_hosting_providers.workspace = true
 gpui = { workspace = true, features = ["test-support"] }
 indoc.workspace = true
 language = { workspace = true, features = ["test-support"] }
+language_model = { workspace = true, features = ["test-support"] }
 live_kit_client = { workspace = true, features = ["test-support"] }
 lsp = { workspace = true, features = ["test-support"] }
 menu.workspace = true