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_model/src/role.rs

@@ -66,6 +66,16 @@ impl From<Role> for open_ai::Role {
     }
 }
 
+impl From<Role> for deepseek::Role {
+    fn from(val: Role) -> Self {
+        match val {
+            Role::User => deepseek::Role::User,
+            Role::Assistant => deepseek::Role::Assistant,
+            Role::System => deepseek::Role::System,
+        }
+    }
+}
+
 impl From<Role> for lmstudio::Role {
     fn from(val: Role) -> Self {
         match val {