Make the app-server thread unload delay configurable (#42320)

## What changed

- Add the top-level `thread_unload_delay_secs` configuration key for the
  app-server, with a 60-second default instead of the previous 30-minute
  fixed delay.
- Allow zero-delay unloading and reject values that cannot fit in a
  monotonic-clock deadline.
- Reset the unload countdown when a thread gains a subscriber or becomes
  active, and preserve active turns even when the configured delay is zero.
- Document that unloading ephemeral threads discards their in-memory state.

## Testing

- Cover configuration parsing, defaults, overrides, and overflow rejection.
- Cover unsubscribe, resubscribe, delayed and immediate unloading,
  notifications, persisted-thread resume, and WebSocket disconnect behavior.

GitOrigin-RevId: edd46f6b49bbdafbf606bff74378e8f569e95977

codex-rs/core/config.schema.json

@@ -6771,6 +6771,12 @@
       "description": "Suppress warnings about unstable (under development) features.",
       "type": "boolean"
     },
+    "thread_unload_delay_secs": {
+      "description": "Seconds a thread must have no subscribers and no activity before app-server unloads it. Defaults to 60; zero unloads immediately. Changes require a server restart.",
+      "format": "uint64",
+      "minimum": 0.0,
+      "type": "integer"
+    },
     "tool_output_token_limit": {
       "description": "Token budget applied when storing tool/function outputs in the context manager.",
       "format": "uint",