agent: Fix crash with pathological fetch output (#34253)

Closes #34029

The crash is due to a stack overflow in our `html_to_markdown`
conversion; I've added a maximum depth of 200 for the recursion in that
crate to guard against this kind of thing.

Separately, we were treating all content-types other than `text/plain`
and `application/json` as HTML; I've changed this to only treat
`text/html` and `application/xhtml+xml` as HTML, and fall back to
plaintext. (In the original crash, the content-type was
`application/octet-stream`.)

Release Notes:

- agent: Fixed a potential crash when fetching large non-HTML files.

crates/html_to_markdown/src/markdown_writer.rs

@@ -119,8 +119,10 @@ impl MarkdownWriter {
                 .push_back(current_element.clone());
         }
 
-        for child in node.children.borrow().iter() {
-            self.visit_node(child, handlers)?;
+        if self.current_element_stack.len() < 200 {
+            for child in node.children.borrow().iter() {
+                self.visit_node(child, handlers)?;
+            }
         }
 
         if let Some(current_element) = current_element {