Report error if FS is incorrectly handled on client filesystem read

Signed-off-by: Marcos Lilljedahl <marcosnils@gmail.com>

tests/plan/client/filesystem/read/fs/invalid_fs_input/test.cue

@@ -0,0 +1,12 @@
+package main
+
+import (
+	"dagger.io/dagger"
+)
+
+dagger.#Plan & {
+	// Reading a file into a dagger.#FS should not be possbile
+	client: filesystem: "../rootfs/test.txt": read: contents: dagger.#FS
+	actions: test: {
+	}
+}