Handle undefined values before JSON serialization in code mode (#43873)

## Why

Passing `undefined` to a tool could fail JSON parsing because V8 stringifies it as the non-JSON text `undefined`.

## What changed

Return no JSON value for JavaScript `undefined`, so explicit `undefined` tool arguments behave like omitted arguments. Attempts to `store` an undefined value report the existing serializability error and preserve the previous stored value.

## Testing

Add regression coverage for storing `undefined` over an existing `null` value, and extend the current-time tool test to cover `{}`, omitted arguments, and explicit `undefined`.

GitOrigin-RevId: 071050ebb2ecc44c8f08453cf8523b4d21728c7e

codex-rs/code-mode-runtime/src/runtime/value.rs

@@ -297,6 +297,11 @@ pub(super) fn v8_value_to_json(
     scope: &mut v8::PinScope<'_, '_>,
     value: v8::Local<'_, v8::Value>,
 ) -> Result<Option<JsonValue>, String> {
+    // V8 stringifies undefined as the non-JSON text "undefined".
+    if value.is_undefined() {
+        return Ok(None);
+    }
+
     let tc = std::pin::pin!(v8::TryCatch::new(scope));
     let mut tc = tc.init();
     let Some(stringified) = v8::json::stringify(&tc, value) else {