Persist session IDs across thread resume (#29327)

## Summary

A cold-resumed subagent kept its durable thread ID but could receive a
new session ID, splitting one agent tree across multiple sessions after
a restart.

Persist the root session ID in every rollout `SessionMeta`, carry it
through thread creation, and restore it before initializing the resumed
`Session` and `AgentControl`.

## Behavior

For a nested agent tree:

```text
root session R
  parent thread P
    child thread C
```

The child rollout stores:

```text
session_id:       R
parent_thread_id: P
id:               C
```

After a cold resume, the child still belongs to root session `R` while
its immediate parent remains `P`. The integration coverage uses distinct
values for all three IDs so it catches restoring the session from
`parent_thread_id`.

## Legacy rollouts

Previous rollouts have `id` but no `session_id`. `SessionMetaLine`
deserialization treats a missing `session_id` as `id`, keeping those
files readable, listable, and resumable. When a legacy subagent is
resumed through its root, that synthesized child ID no longer overrides
the inherited root-scoped `AgentControl`. New rollouts always persist
the explicit root session ID.

codex-rs/protocol/src/protocol.rs

@@ -57,7 +57,9 @@ use codex_utils_absolute_path::AbsolutePathBuf;
 use codex_utils_path_uri::PathUri;
 use schemars::JsonSchema;
 use serde::Deserialize;
+use serde::Deserializer;
 use serde::Serialize;
+use serde::de::Error as _;
 use serde_json::Value;
 use serde_with::serde_as;
 use strum_macros::Display;
@@ -2914,6 +2916,7 @@ pub enum MultiAgentVersion {
 /// and should be used when there is no config override.
 #[derive(Serialize, Deserialize, Clone, Debug, JsonSchema, TS)]
 pub struct SessionMeta {
+    pub session_id: SessionId,
     pub id: ThreadId,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub forked_from_id: Option<ThreadId>,
@@ -2956,8 +2959,10 @@ pub struct SessionMeta {
 
 impl Default for SessionMeta {
     fn default() -> Self {
+        let id = ThreadId::default();
         SessionMeta {
-            id: ThreadId::default(),
+            session_id: id.into(),
+            id,
             forked_from_id: None,
             parent_thread_id: None,
             timestamp: String::new(),
@@ -2978,7 +2983,7 @@ impl Default for SessionMeta {
     }
 }
 
-#[derive(Serialize, Deserialize, Debug, Clone, JsonSchema, TS)]
+#[derive(Serialize, Debug, Clone, JsonSchema, TS)]
 pub struct SessionMetaLine {
     #[serde(flatten)]
     pub meta: SessionMeta,
@@ -2986,6 +2991,35 @@ pub struct SessionMetaLine {
     pub git: Option<GitInfo>,
 }
 
+impl<'de> Deserialize<'de> for SessionMetaLine {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        #[derive(Deserialize)]
+        struct SessionMetaLineFields {
+            #[serde(flatten)]
+            meta: SessionMeta,
+            git: Option<GitInfo>,
+        }
+
+        let mut value = Value::deserialize(deserializer)?;
+        let fields = value
+            .as_object_mut()
+            .ok_or_else(|| D::Error::custom("session metadata must be an object"))?;
+        if !fields.contains_key("session_id") {
+            let thread_id = fields
+                .get("id")
+                .cloned()
+                .ok_or_else(|| D::Error::missing_field("id"))?;
+            fields.insert("session_id".to_string(), thread_id);
+        }
+        let SessionMetaLineFields { meta, git } =
+            serde_json::from_value(value).map_err(D::Error::custom)?;
+        Ok(Self { meta, git })
+    }
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone, JsonSchema, TS)]
 #[serde(tag = "type", content = "payload", rename_all = "snake_case")]
 pub enum RolloutItem {
@@ -5390,6 +5424,7 @@ mod tests {
         let thread_id = ThreadId::from_string("67e55044-10b1-426f-9247-bb680e5fe0c8")?;
         let older_meta = SessionMetaLine {
             meta: SessionMeta {
+                session_id: thread_id.into(),
                 id: thread_id,
                 multi_agent_version: Some(MultiAgentVersion::V2),
                 ..Default::default()
@@ -5398,6 +5433,7 @@ mod tests {
         };
         let newer_meta_without_version = SessionMetaLine {
             meta: SessionMeta {
+                session_id: thread_id.into(),
                 id: thread_id,
                 multi_agent_version: None,
                 ..Default::default()