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/unified_exec/mod.rs

@@ -155,6 +155,8 @@ struct ProcessEntry {
     process: Arc<UnifiedExecProcess>,
     call_id: String,
     process_id: i32,
+    cwd: AbsolutePathBuf,
+    initial_exec_command_active: Arc<std::sync::atomic::AtomicBool>,
     hook_command: String,
     tty: bool,
     network_approval: Option<DeferredNetworkApproval>,