Drop whitespace-only content nodes from parser output

Content between tags (e.g. newlines between </think> and <tool_call>)
was creating empty Content nodes. Now trimmed before creating the node.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/agent/context.rs

@@ -623,7 +623,7 @@ impl ResponseParser {
     fn flush_content(&mut self, ctx: &mut ContextState) {
         if !self.content_parts.is_empty() {
             let text: String = self.content_parts.drain(..).collect();
-            if !text.is_empty() {
+            if !text.trim().is_empty() {
                 self.push_child(ctx, AstNode::content(text));
             }
         }