Move environment abstraction into exec server (#15125)

The idea is that codex-exec exposes an Environment struct with services
on it. Each of those is a trait.

Depending on construction parameters passed to Environment they are
either backed by local or remote server but core doesn't see these
differences.

codex-rs/core/src/tools/handlers/view_image.rs

@@ -1,5 +1,5 @@
 use async_trait::async_trait;
-use codex_environment::ExecutorFileSystem;
+use codex_exec_server::ExecutorFileSystem;
 use codex_protocol::models::FunctionCallOutputBody;
 use codex_protocol::models::FunctionCallOutputContentItem;
 use codex_protocol::models::FunctionCallOutputPayload;