[ty] Fix bug #879 in signature help (#19542)

This PR fixes bug [#879](astral-sh/ty#879)
where the signature help popup remains visible after typing the closing
paren in a call expression.

Co-authored-by: UnboundVariable <[email protected]>

Author: UnboundVariable

crates/ty_ide/src/signature_help.rs

@@ -66,6 +66,10 @@ pub fn signature_help(db: &dyn Db, file: File, offset: TextSize) -> Option<Signa
     // Get the call expression at the given position.
     let (call_expr, current_arg_index) = get_call_expr(&parsed, offset)?;
 
+    if offset >= call_expr.end() {
+        return None;
+    }
+
     // Get signature details from the semantic analyzer.
     let signature_details: Vec<CallSignatureDetails<'_>> =
         call_signature_details(db, file, call_expr);