Reject dynamic environments named local (#34784)

## Why

`local` identifies the environment owned by `EnvironmentManager`, but dynamic
remote environment registration previously allowed that ID to be replaced.

## What changed

Reject `local` in the shared environment ID validator used by dynamic remote
environment registration.

GitOrigin-RevId: 2622551f78989c16c432e6dd3d7da9bd8b0527c5

codex-rs/exec-server/src/environment.rs

@@ -490,6 +490,11 @@ fn validate_environment_id(environment_id: &str) -> Result<(), ExecServerError>
             "environment id cannot be empty".to_string(),
         ));
     }
+    if environment_id == LOCAL_ENVIRONMENT_ID {
+        return Err(ExecServerError::Protocol(format!(
+            "environment id `{LOCAL_ENVIRONMENT_ID}` is reserved for EnvironmentManager"
+        )));
+    }
     Ok(())
 }