Fix Thinking/Log panics: skip entries with empty token_ids

Entries with empty token_ids (Thinking, Log) are not part of the
prompt and don't have messages. Skip them in streaming_index(),
route_entry(), and sync_from_agent() instead of calling .message()
which panics.

Using token_ids.is_empty() as the guard in streaming_index means
the check is tied to the data, not the type — any entry that
doesn't produce tokens is safely skipped.

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

src/agent/mod.rs

@@ -370,6 +370,7 @@ impl Agent {
     /// Find the index of the in-progress streaming entry (unstamped assistant message).
     fn streaming_index(&self) -> Option<usize> {
         self.context.conversation.entries().iter().rposition(|ce| {
+            if ce.token_ids.is_empty() { return false; }
             let m = ce.entry.message();
             m.role == Role::Assistant && m.timestamp.is_none()
         })