agent: Fix an issue with messages containing trailing whitespace (#33643)

Seeing this come up in our server logs when sending requests to
Anthropic: `final assistant content cannot end with trailing
whitespace`.


Release Notes:

- agent: Fixed an issue where Anthropic requests would sometimes fail
because of malformed assistant messages

crates/agent/src/thread.rs

@@ -1343,6 +1343,7 @@ impl Thread {
             for segment in &message.segments {
                 match segment {
                     MessageSegment::Text(text) => {
+                        let text = text.trim_end();
                         if !text.is_empty() {
                             request_message
                                 .content