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/semantic_index/src/semantic_index.rs

@@ -1263,4 +1263,4 @@ mod tests {
 }
 
 // See https://github.com/zed-industries/zed/pull/14823#discussion_r1684616398 for why this is here and when it should be removed.
-type _TODO = completion::CompletionProvider;
+type _TODO = completion::LanguageModelCompletionProvider;