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

@@ -5,6 +5,7 @@ import (
 )
 
 dagger.#Plan & {
+	// Reading a file into a dagger.#FS should not be possbile
 	client: filesystem: "../rootfs/test.txt": read: contents: dagger.#FS
 	actions: test: {
 	}

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

@@ -0,0 +1,21 @@
+package main
+
+import (
+	"dagger.io/dagger"
+	"dagger.io/dagger/core"
+)
+
+dagger.#Plan & {
+	// Reading a directory into a non-fs should fail
+	client: filesystem: "../rootfs": read: contents: string
+	actions: {
+		image: core.#Pull & {
+			source: "alpine:3.15.0@sha256:e7d88de73db3d3fd9b2d63aa7f447a10fd0220b7cbf39803c803f2af9ba256b3"
+		}
+		test: core.#Exec & {
+			input: image.output
+			args: ["test", client.filesystem."../rootfs".read.contents]
+		}
+
+	}
+}