Reduce the default thread unload delay to one minute (#57242)

## What changed

Change the default `thread_unload_delay_secs` from 1800 to 60 seconds. App-server threads become eligible for unloading after one minute with no subscribers and no activity. Update the configuration documentation, schema, and thread manager sample to match.

## Testing

Update the configuration test to cover the 60-second default, zero for immediate unloading, and an explicit 1800-second override.

GitOrigin-RevId: 0da8454eb25a6ef4b7e3665601982d2e0c183f7c

codex-rs/thread-manager-sample/src/main.rs

@@ -383,7 +383,7 @@ async fn new_config(
         tool_registry: Default::default(),
         code_mode: Default::default(),
         background_terminal_max_timeout: 300_000,
-        thread_unload_delay: std::time::Duration::from_secs(/*secs*/ 1800),
+        thread_unload_delay: std::time::Duration::from_secs(/*secs*/ 60),
         ghost_snapshot: GhostSnapshotConfig::default(),
         multi_agent_v2: MultiAgentV2Config::default(),
         max_goal_token_budget: None,