Add app-server background terminal process APIs (#26041)

## Summary

Codex Apps needs app-server as the source of truth for chat-started
background terminals instead of guessing from local process trees.

This PR adds experimental v2 APIs to list and terminate background
terminals for a loaded thread using app-server process ids, so clients
can manage background terminals without local PID discovery.

## Changes

- `thread/backgroundTerminals/list` returns paginated background
terminal records with `itemId`, app-server `processId`, `command`,
`cwd`, nullable `osPid`, nullable `cpuPercent`, and nullable `rssKb`.
- `thread/backgroundTerminals/terminate` terminates one running
background terminal by app-server `processId` and returns whether a
process was terminated.
- Background terminal list and terminate operations use unified-exec
process manager state as their source of truth.

codex-rs/core/src/codex_thread.rs

@@ -159,6 +159,14 @@ pub struct CodexThread {
     out_of_band_elicitation_count: Mutex<u64>,
 }
 
+#[derive(Debug, Eq, PartialEq)]
+pub struct BackgroundTerminalInfo {
+    pub item_id: String,
+    pub process_id: String,
+    pub command: String,
+    pub cwd: AbsolutePathBuf,
+}
+
 /// Conduit for the bidirectional stream of messages that compose a thread
 /// (formerly called a conversation) in Codex.
 impl CodexThread {
@@ -396,6 +404,17 @@ impl CodexThread {
         self.codex.agent_status().await
     }
 
+    pub async fn list_background_terminals(&self) -> Vec<BackgroundTerminalInfo> {
+        self.codex.session.list_background_terminals().await
+    }
+
+    pub async fn terminate_background_terminal(&self, process_id: i32) -> bool {
+        self.codex
+            .session
+            .terminate_background_terminal(process_id)
+            .await
+    }
+
     pub(crate) fn subscribe_status(&self) -> watch::Receiver<AgentStatus> {
         self.codex.agent_status.clone()
     }