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/file_system_handler.rs

@@ -33,6 +33,8 @@ 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::rpc::internal_error;
@@ -198,6 +200,16 @@ impl FileSystemHandler {
         Ok(FsReadDirectoryResponse { entries })
     }
 
+    pub(crate) async fn walk(
+        &self,
+        params: FsWalkParams,
+    ) -> Result<FsWalkResponse, JSONRPCErrorError> {
+        self.file_system
+            .walk(&params.path, params.options, params.sandbox.as_ref())
+            .await
+            .map_err(map_fs_error)
+    }
+
     pub(crate) async fn remove(
         &self,
         params: FsRemoveParams,

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,

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

@@ -17,6 +17,7 @@ use crate::protocol::FS_READ_BLOCK_METHOD;
 use crate::protocol::FS_READ_DIRECTORY_METHOD;
 use crate::protocol::FS_READ_FILE_METHOD;
 use crate::protocol::FS_REMOVE_METHOD;
+use crate::protocol::FS_WALK_METHOD;
 use crate::protocol::FS_WRITE_FILE_METHOD;
 use crate::protocol::FsCanonicalizeParams;
 use crate::protocol::FsCloseParams;
@@ -28,6 +29,7 @@ use crate::protocol::FsReadBlockParams;
 use crate::protocol::FsReadDirectoryParams;
 use crate::protocol::FsReadFileParams;
 use crate::protocol::FsRemoveParams;
+use crate::protocol::FsWalkParams;
 use crate::protocol::FsWriteFileParams;
 use crate::protocol::HTTP_REQUEST_METHOD;
 use crate::protocol::HttpRequestParams;
@@ -147,6 +149,12 @@ pub(crate) fn build_router() -> RpcRouter<ExecServerHandler> {
             handler.fs_read_directory(params).await
         },
     );
+    router.request(
+        FS_WALK_METHOD,
+        |handler: Arc<ExecServerHandler>, params: FsWalkParams| async move {
+            handler.fs_walk(params).await
+        },
+    );
     router.request(
         FS_REMOVE_METHOD,
         |handler: Arc<ExecServerHandler>, params: FsRemoveParams| async move {