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/common.rs

@@ -1184,7 +1184,8 @@ client_request_definitions! {
 macro_rules! server_request_definitions {
     (
         $(
-            $(#[$variant_meta:meta])*
+            $(#[experimental($reason:expr)])?
+            $(#[doc = $variant_doc:literal])*
             $variant:ident $(=> $wire:literal)? {
                 params: $params:ty,
                 response: $response:ty,
@@ -1197,7 +1198,7 @@ macro_rules! server_request_definitions {
         #[serde(tag = "method", rename_all = "camelCase")]
         pub enum ServerRequest {
             $(
-                $(#[$variant_meta])*
+                $(#[doc = $variant_doc])*
                 $(#[serde(rename = $wire)] #[ts(rename = $wire)])?
                 $variant {
                     #[serde(rename = "id")]
@@ -1237,7 +1238,7 @@ macro_rules! server_request_definitions {
         #[serde(tag = "method", rename_all = "camelCase")]
         pub enum ServerResponse {
             $(
-                $(#[$variant_meta])*
+                $(#[doc = $variant_doc])*
                 $(#[serde(rename = $wire)])?
                 $variant {
                     #[serde(rename = "id")]
@@ -1281,6 +1282,22 @@ macro_rules! server_request_definitions {
             }
         }
 
+        pub(crate) const EXPERIMENTAL_SERVER_METHODS: &[&str] = &[
+            $(
+                experimental_method_entry!($(#[experimental($reason)])? $(=> $wire)?),
+            )*
+        ];
+        pub(crate) const EXPERIMENTAL_SERVER_METHOD_PARAM_TYPES: &[&str] = &[
+            $(
+                experimental_type_entry!($(#[experimental($reason)])? $params),
+            )*
+        ];
+        pub(crate) const EXPERIMENTAL_SERVER_METHOD_RESPONSE_TYPES: &[&str] = &[
+            $(
+                experimental_type_entry!($(#[experimental($reason)])? $response),
+            )*
+        ];
+
         pub fn export_server_responses(
             out_dir: &::std::path::Path,
         ) -> ::std::result::Result<(), ::ts_rs::ExportError> {
@@ -1470,6 +1487,13 @@ server_request_definitions! {
         response: v2::AttestationGenerateResponse,
     },
 
+    #[experimental("currentTime/read")]
+    /// Read the current time from an external clock owned by the client.
+    CurrentTimeRead => "currentTime/read" {
+        params: v2::CurrentTimeReadParams,
+        response: v2::CurrentTimeReadResponse,
+    },
+
     /// DEPRECATED APIs below
     /// Request to approve a patch.
     /// This request is used for Turns started via the legacy APIs (i.e. SendUserTurn, SendUserMessage).
@@ -2372,6 +2396,32 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn serialize_current_time_read_request() -> Result<()> {
+        let params = v2::CurrentTimeReadParams {
+            thread_id: "thread-123".to_string(),
+        };
+        let request = ServerRequest::CurrentTimeRead {
+            request_id: RequestId::Integer(10),
+            params: params.clone(),
+        };
+        assert_eq!(
+            json!({
+                "method": "currentTime/read",
+                "id": 10,
+                "params": {
+                    "threadId": "thread-123"
+                }
+            }),
+            serde_json::to_value(&request)?,
+        );
+
+        let payload = ServerRequestPayload::CurrentTimeRead(params);
+        assert_eq!(request.id(), &RequestId::Integer(10));
+        assert_eq!(payload.request_with_id(RequestId::Integer(10)), request);
+        Ok(())
+    }
+
     #[test]
     fn serialize_server_response() -> Result<()> {
         let response = ServerResponse::CommandExecutionRequestApproval {

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,
+}

codex-rs/app-server-protocol/src/protocol/v2/mod.rs

@@ -6,6 +6,7 @@ mod attestation;
 mod collaboration_mode;
 mod command_exec;
 mod config;
+mod current_time;
 mod environment;
 mod experimental_feature;
 mod feedback;
@@ -32,6 +33,7 @@ pub use attestation::*;
 pub use collaboration_mode::*;
 pub use command_exec::*;
 pub use config::*;
+pub use current_time::*;
 pub use environment::*;
 pub use experimental_feature::*;
 pub use feedback::*;