Refactor ExecServer filesystem split between local and remote (#15232)

For each feature we have:
1. Trait exposed on environment
2. **Local Implementation** of the trait
3. Remote implementation that uses the client to proxy via network
4. Handler implementation that handles PRC requests and calls into
**Local Implementation**

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

@@ -1,5 +1,4 @@
 use async_trait::async_trait;
-use codex_exec_server::ExecutorFileSystem;
 use codex_protocol::models::FunctionCallOutputBody;
 use codex_protocol::models::FunctionCallOutputContentItem;
 use codex_protocol::models::FunctionCallOutputPayload;