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/message_processor.rs

@@ -7,6 +7,7 @@ use std::sync::atomic::AtomicBool;
 use crate::attestation::app_server_attestation_provider;
 use crate::config_manager::ConfigManager;
 use crate::connection_rpc_gate::ConnectionRpcGate;
+use crate::current_time::app_server_time_provider;
 use crate::error_code::invalid_request;
 use crate::extensions::ThreadExtensionDependencies;
 use crate::extensions::app_server_extension_event_sink;
@@ -280,7 +281,6 @@ impl ConnectionSessionState {
             .get()
             .is_some_and(|session| session.supports_openai_form_elicitation)
     }
-
     pub(crate) fn initialize(&self, session: InitializedConnectionSessionState) -> Result<(), ()> {
         self.initialized.set(session).map_err(|_| ())
     }
@@ -379,7 +379,10 @@ impl MessageProcessor {
                     outgoing.clone(),
                     thread_state_manager.clone(),
                 )),
-                /*external_time_provider*/ None,
+                Some(app_server_time_provider(
+                    outgoing.clone(),
+                    thread_state_manager.clone(),
+                )),
             )
         });
         let models_manager = thread_manager.get_models_manager();