context_server: Fix arguments handling (#17478)

We accidentally do not accept prompts with an empty list of arguments,
as opposed to non given arguments list. We need to allow these. We also
not really supporting non required arguments, despite the protocol
describing it. This is a first iteration on fixing this as well.

Release Notes:

- N/A

crates/assistant/src/slash_command/context_server_command.rs

@@ -44,10 +44,9 @@ impl SlashCommand for ContextServerSlashCommand {
     }
 
     fn requires_argument(&self) -> bool {
-        self.prompt
-            .arguments
-            .as_ref()
-            .map_or(false, |args| !args.is_empty())
+        self.prompt.arguments.as_ref().map_or(false, |args| {
+            args.iter().any(|arg| arg.required == Some(true))
+        })
     }
 
     fn complete_argument(
@@ -179,6 +178,8 @@ fn prompt_arguments(prompt: &PromptInfo, arguments: &[String]) -> Result<HashMap
                 let mut map = HashMap::default();
                 map.insert(args[0].name.clone(), arguments.join(" "));
                 Ok(map)
+            } else if arguments.is_empty() && args[0].required == Some(false) {
+                Ok(HashMap::default())
             } else {
                 Err(anyhow!("Prompt expects argument but none given"))
             }
@@ -199,7 +200,7 @@ fn prompt_arguments(prompt: &PromptInfo, arguments: &[String]) -> Result<HashMap
 pub fn acceptable_prompt(prompt: &PromptInfo) -> bool {
     match &prompt.arguments {
         None => true,
-        Some(args) if args.len() == 1 => true,
+        Some(args) if args.len() <= 1 => true,
         _ => false,
     }
 }