Add GitHub Copilot Chat Support (#14842)

# Summary

This commit implements Github Copilot Chat support within the existing
Assistant panel/framework. It required a little bit of trickery and
internal API modification, as Copilot doesn't use the same
authentication-style as all of the existing providers, opting to use
OAuth and a short lived API key instead of a straight API key. All
existing Assistant features should work.

Release Notes:
- Added Github Copilot Chat support
([#4673](https://github.com/zed-industries/zed/issues/4673)).

## Screenshots
<img width="1552" alt="A screenshot showing a conversation between a
user and Github Copilot Chat within the Zed editor."
src="https://github.com/user-attachments/assets/73eaf6a2-792b-4c40-a7fe-f763bd6417d7">

---------

Co-authored-by: Bennet Bo Fenner <bennet@zed.dev>

crates/language_model/src/registry.rs

@@ -1,8 +1,8 @@
 use crate::{
     provider::{
         anthropic::AnthropicLanguageModelProvider, cloud::CloudLanguageModelProvider,
-        google::GoogleLanguageModelProvider, ollama::OllamaLanguageModelProvider,
-        open_ai::OpenAiLanguageModelProvider,
+        copilot_chat::CopilotChatLanguageModelProvider, google::GoogleLanguageModelProvider,
+        ollama::OllamaLanguageModelProvider, open_ai::OpenAiLanguageModelProvider,
     },
     LanguageModel, LanguageModelProvider, LanguageModelProviderId, LanguageModelProviderState,
 };
@@ -44,6 +44,7 @@ fn register_language_model_providers(
         GoogleLanguageModelProvider::new(client.http_client(), cx),
         cx,
     );
+    registry.register_provider(CopilotChatLanguageModelProvider::new(cx), cx);
 
     cx.observe_flag::<feature_flags::LanguageModels, _>(move |enabled, cx| {
         let client = client.clone();