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/api/types.rs

@@ -10,7 +10,7 @@ use chrono::Utc;
 use serde::{Deserialize, Serialize};
 
 /// Function call within a tool call — name + JSON arguments.
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 pub struct FunctionCall {
     pub name: String,
     pub arguments: String,
@@ -24,7 +24,7 @@ pub struct FunctionCallDelta {
 }
 
 /// A tool call requested by the model.
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 pub struct ToolCall {
     pub id: String,
     #[serde(rename = "type")]
@@ -45,7 +45,7 @@ pub struct ToolCallDelta {
 /// Message content — either plain text or an array of content parts
 /// (for multimodal messages with images). Serializes as a JSON string
 /// for text-only, or a JSON array for multimodal.
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 #[serde(untagged)]
 pub enum MessageContent {
     Text(String),
@@ -70,7 +70,7 @@ impl MessageContent {
 }
 
 /// A single content part within a multimodal message.
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 #[serde(tag = "type")]
 pub enum ContentPart {
     #[serde(rename = "text")]
@@ -80,13 +80,13 @@ pub enum ContentPart {
 }
 
 /// Image URL — either a real URL or a base64 data URI.
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 pub struct ImageUrl {
     pub url: String,
 }
 
 /// A chat message in the conversation.
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 pub struct Message {
     pub role: Role,
     pub content: Option<MessageContent>,

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 },