chat: PartialEq on ConversationEntry for proper diff

Add PartialEq to Message, FunctionCall, ToolCall, ContentPart,
ImageUrl, MessageContent, ConversationEntry. Sync now compares
entries directly instead of content lengths.

Phase 1 pops mismatched tail entries using PartialEq comparison.
Phase 2 pushes new entries with clone into last_entries buffer.

TODO: route_entry needs to handle multiple tool calls per entry.

Co-Authored-By: Kent Overstreet <kent.overstreet@linux.dev>

src/agent/context.rs

@@ -149,7 +149,7 @@ pub fn is_stream_error(err: &anyhow::Error) -> bool {
 
 /// Conversation entry — either a regular message or memory content.
 /// Memory entries preserve the original message for KV cache round-tripping.
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, PartialEq)]
 pub enum ConversationEntry {
     Message(Message),
     Memory { key: String, message: Message },