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/app-server/src/message_processor.rs

@@ -1126,6 +1126,16 @@ impl MessageProcessor {
                     .thread_background_terminals_clean(&request_id, params)
                     .await
             }
+            ClientRequest::ThreadBackgroundTerminalsList { params, .. } => {
+                self.thread_processor
+                    .thread_background_terminals_list(params)
+                    .await
+            }
+            ClientRequest::ThreadBackgroundTerminalsTerminate { params, .. } => {
+                self.thread_processor
+                    .thread_background_terminals_terminate(params)
+                    .await
+            }
             ClientRequest::ThreadRollback { params, .. } => {
                 self.thread_processor
                     .thread_rollback(&request_id, params)