Migrate fork and resume reads to thread store (#18900)

- Route cold thread/resume and thread/fork source loading through
ThreadStore reads instead of direct rollout path operations
- Keep lookups that explicitly specify a rollout-path using the local
thread store methods but return an invalid-request error for remote
ThreadStore configurations
- Add some additional unit tests for code path coverage

codex-rs/protocol/src/protocol.rs

@@ -2640,7 +2640,7 @@ pub struct ConversationPathResponseEvent {
 pub struct ResumedHistory {
     pub conversation_id: ThreadId,
     pub history: Vec<RolloutItem>,
-    pub rollout_path: PathBuf,
+    pub rollout_path: Option<PathBuf>,
 }
 
 #[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS)]