Advertise environment config read support (#37654)

## What changed

- Add `environmentConfigRead` to exec-server environment capabilities and advertise it for local executors.
- Default the capability to `false` when deserializing responses from older executors.

## Testing

- Verify legacy capability responses remain compatible and the environment config integration reports support.

GitOrigin-RevId: c2110fea9e0e6e756eaeaaad096c50a943668c99

codex-rs/exec-server/tests/environment_config.rs

@@ -44,6 +44,9 @@ unselected = "do not return"
     .await?;
 
     let environment = Environment::create_for_tests(Some(server.websocket_url().to_string()))?;
+    let environment_info = environment.info().await?;
+    assert!(environment_info.capabilities.environment_config_read);
+
     let response = environment
         .read_environment_config(EnvironmentConfigReadParams {
             cwd: PathUri::from_abs_path(&project),