agent: Add tests for thread serialization code (#32383)

This adds some unit tests to ensure that the `update(...)`/migration
path to the latest versions works correctly

Release Notes:

- N/A

crates/agent/src/thread.rs

@@ -195,20 +195,20 @@ impl MessageSegment {
     }
 }
 
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
 pub struct ProjectSnapshot {
     pub worktree_snapshots: Vec<WorktreeSnapshot>,
     pub unsaved_buffer_paths: Vec<String>,
     pub timestamp: DateTime<Utc>,
 }
 
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
 pub struct WorktreeSnapshot {
     pub worktree_path: String,
     pub git_state: Option<GitState>,
 }
 
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
 pub struct GitState {
     pub remote_url: Option<String>,
     pub head_sha: Option<String>,
@@ -247,7 +247,7 @@ impl LastRestoreCheckpoint {
     }
 }
 
-#[derive(Clone, Debug, Default, Serialize, Deserialize)]
+#[derive(Clone, Debug, Default, Serialize, Deserialize, PartialEq)]
 pub enum DetailedSummaryState {
     #[default]
     NotGenerated,
@@ -391,7 +391,7 @@ impl ThreadSummary {
     }
 }
 
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
 pub struct ExceededWindowError {
     /// Model used when last message exceeded context window
     model_id: LanguageModelId,