Add DeepSeek support (#23551)

- Added support for DeepSeek as a new language model provider in Zed
Assistant
- Implemented streaming API support for real-time responses from
DeepSeek models.
- Added a configuration UI for DeepSeek API key management and settings.
- Updated documentation with detailed setup instructions for DeepSeek
integration.
- Added DeepSeek-specific icons and model definitions for seamless
integration into the Zed UI.
- Integrated DeepSeek into the language model registry, making it
available alongside other providers like OpenAI and Anthropic.

Release Notes:

- Added support for DeepSeek to the Assistant.

---------

Co-authored-by: Marshall Bowers <git@maxdeviant.com>

crates/language_models/src/language_models.rs

@@ -4,6 +4,7 @@ use client::{Client, UserStore};
 use fs::Fs;
 use gpui::{App, Context, Entity};
 use language_model::{LanguageModelProviderId, LanguageModelRegistry, ZED_CLOUD_PROVIDER_ID};
+use provider::deepseek::DeepSeekLanguageModelProvider;
 
 mod logging;
 pub mod provider;
@@ -55,6 +56,10 @@ fn register_language_model_providers(
         LmStudioLanguageModelProvider::new(client.http_client(), cx),
         cx,
     );
+    registry.register_provider(
+        DeepSeekLanguageModelProvider::new(client.http_client(), cx),
+        cx,
+    );
     registry.register_provider(
         GoogleLanguageModelProvider::new(client.http_client(), cx),
         cx,