Make thread item timestamps optional in TypeScript exports (#57420)

## Why

Older servers can omit thread item timestamps. The TypeScript contract should accept those responses while still allowing explicit `null` values.

## What changed

Mark `ThreadItem.startedAtMs` and `ThreadItem.completedAtMs` as optional nullable fields. Update the generated TypeScript and precomputed stable and experimental exports, and allow these fields in the optional-nullable export check.

GitOrigin-RevId: 186be7aa45a3e7c65f133b1e320809a9912ff1f3

codex-rs/app-server-protocol/src/export.rs

@@ -2411,6 +2411,8 @@ mod tests {
                     && field_prefix.trim() == "threadUsage?")
                     || (path == Path::new("v2/ThreadUsage.ts")
                         && field_prefix.trim() == "nativeUsageUsdMicros?")
+                    || (path == Path::new("v2/ThreadItem.ts")
+                        && matches!(field_prefix.trim(), "startedAtMs?" | "completedAtMs?"))
                     || (path == Path::new("v2/McpServerOauthLoginCompletedNotification.ts")
                         && field_prefix.trim() == "loginId?")
                     || (path == Path::new("v2/MisalignmentErrorDetails.ts")

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

@@ -240,11 +240,11 @@ pub struct ThreadItem {
     pub payload: ThreadItemPayload,
     /// Unix timestamp in milliseconds when the item started, if recorded.
     #[serde(default)]
-    #[ts(type = "number | null")]
+    #[ts(type = "number | null", optional)]
     pub started_at_ms: Option<i64>,
     /// Unix timestamp in milliseconds when the item completed, if recorded.
     #[serde(default)]
-    #[ts(type = "number | null")]
+    #[ts(type = "number | null", optional)]
     pub completed_at_ms: Option<i64>,
 }