Add app-server current-time impl (varlatency 3/n) (#28835)

## What

Server should request:

```
{
  "id": 42,
  "method": "currentTime/read",
  "params": {
    "threadId": "11111111-1111-1111-1111-aaaaafdc2c11"
  }
}
```

Client should respond with something like:

```rust
{
  "id": 42,
  "result": {
    "currentTimeAt": 1781717655
  }
}
```

## Why

Sessions configured with `clock_source = "external"` need a
thread-specific external time source before inference. The system clock
remains the default production provider.

## Validation

- `cargo test -p codex-app-server-protocol`
- `cargo test -p codex-app-server --test all
current_time_read_round_trip_adds_reminder_to_model_input`
- `cargo test -p codex-app-server
first_attestation_capable_connection_for_thread_only_uses_thread_subscribers`
- `cargo test -p codex-analytics`
- `just fix -p codex-app-server-protocol`
- `just fix -p codex-app-server`

Stacked on #28824.

codex-rs/app-server/src/thread_state.rs

@@ -329,6 +329,23 @@ impl ThreadStateManager {
             .min_by_key(|connection_id| connection_id.0)
     }
 
+    pub(crate) async fn wait_for_thread_subscriber(&self, thread_id: ThreadId) {
+        let mut has_connections = {
+            let mut state = self.state.lock().await;
+            state
+                .threads
+                .entry(thread_id)
+                .or_default()
+                .has_connections_watcher
+                .subscribe()
+        };
+        while !*has_connections.borrow_and_update() {
+            if has_connections.changed().await.is_err() {
+                break;
+            }
+        }
+    }
+
     pub(crate) async fn subscribed_connection_ids(&self, thread_id: ThreadId) -> Vec<ConnectionId> {
         let state = self.state.lock().await;
         state