Add a bounded filesystem walk RPC (#29841)

Stack 1 of 3. Follow-ups: #29842 and #29844.

## What changes

Adds a general bounded `fs/walk` operation to the exec server.

The operation returns file and directory entries plus recoverable
per-path errors. It skips symlinks, preserves the existing filesystem
sandbox routing, and enforces depth, directory, entry, and response-size
limits.

This PR only defines and wires the filesystem operation. It does not
change any callers yet.

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

@@ -37,6 +37,8 @@ use crate::protocol::FsReadFileParams;
 use crate::protocol::FsReadFileResponse;
 use crate::protocol::FsRemoveParams;
 use crate::protocol::FsRemoveResponse;
+use crate::protocol::FsWalkParams;
+use crate::protocol::FsWalkResponse;
 use crate::protocol::FsWriteFileParams;
 use crate::protocol::FsWriteFileResponse;
 use crate::protocol::HttpRequestParams;
@@ -311,6 +313,14 @@ impl ExecServerHandler {
         self.file_system.read_directory(params).await
     }
 
+    pub(crate) async fn fs_walk(
+        &self,
+        params: FsWalkParams,
+    ) -> Result<FsWalkResponse, JSONRPCErrorError> {
+        self.require_initialized_for("filesystem")?;
+        self.file_system.walk(params).await
+    }
+
     pub(crate) async fn fs_remove(
         &self,
         params: FsRemoveParams,