context_servers: Completion support for context server slash commands (#17085)

This PR adds support for completions via MCP. The protocol now supports
a new request type "completion/complete"
that can either complete a resource URI template (which we currently
don't use in Zed), or a prompt argument.
We use this to add autocompletion to our context server slash commands!


https://github.com/user-attachments/assets/08c9cf04-cbeb-49a7-903f-5049fb3b3d9f



Release Notes:

- context_servers: Added support for argument completions for context
server prompts. These show up as regular completions to slash commands.

crates/context_servers/src/protocol.rs

@@ -127,6 +127,35 @@ impl InitializedContextServerProtocol {
 
         Ok(response)
     }
+
+    pub async fn completion<P: Into<String>>(
+        &self,
+        reference: types::CompletionReference,
+        argument: P,
+        value: P,
+    ) -> Result<types::Completion> {
+        let params = types::CompletionCompleteParams {
+            r#ref: reference,
+            argument: types::CompletionArgument {
+                name: argument.into(),
+                value: value.into(),
+            },
+        };
+        let result: types::CompletionCompleteResponse = self
+            .inner
+            .request(types::RequestType::CompletionComplete.as_str(), params)
+            .await?;
+
+        let completion = types::Completion {
+            values: result.completion.values,
+            total: types::CompletionTotal::from_options(
+                result.completion.has_more,
+                result.completion.total,
+            ),
+        };
+
+        Ok(completion)
+    }
 }
 
 impl InitializedContextServerProtocol {