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/exec/src/lib.rs

@@ -1719,6 +1719,15 @@ async fn handle_server_request(
             )
             .await
         }
+        ServerRequest::CurrentTimeRead { request_id, .. } => {
+            reject_server_request(
+                client,
+                request_id,
+                &method,
+                "external current time is not supported in exec mode".to_string(),
+            )
+            .await
+        }
         ServerRequest::ApplyPatchApproval { request_id, params } => {
             reject_server_request(
                 client,