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-protocol/src/protocol/v2/current_time.rs

@@ -0,0 +1,20 @@
+use schemars::JsonSchema;
+use serde::Deserialize;
+use serde::Serialize;
+use ts_rs::TS;
+
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct CurrentTimeReadParams {
+    pub thread_id: String,
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct CurrentTimeReadResponse {
+    /// Current time as whole Unix seconds.
+    #[ts(type = "number")]
+    pub current_time_at: i64,
+}