refactor(tvix/castore): add try_into_anonymous_node, rename to try_*

We have two places where we parse protos and want their names to be
empty:

 - Receiving a root node in a nar-bridge NAR request
 - Processing the CalculateNAR gRPC call

We don't have any place where we want to keep a name as bytes::Bytes
around, yet we used the `into_name_bytes_and_node` method.

It was also a bit wrongly named - it wasn't very clear the name was
not validated, and that the function may fail.

This moves the "splitting off the name as bytes::Bytes" part into a
private helper, only leaving the `try_into_name_and_node` and
`try_into_anonymous_node` methods around.

Change-Id: I2c7fd9871d49ec67450d7efa6a30d96197fb319c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12664
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: Marijan Petričević <marijan.petricevic94@gmail.com>
Reviewed-by: raitobezarius <tvl@lahfa.xyz>

tvix/castore/src/proto/tests/mod.rs

@@ -1,4 +1,5 @@
 use super::{node, Node, SymlinkNode};
+use crate::DirectoryError;
 
 mod directory;
 
@@ -11,7 +12,7 @@ fn convert_symlink_empty_target_invalid() {
             target: "".into(),
         })),
     }
-    .into_name_and_node()
+    .try_into_name_and_node()
     .expect_err("must fail validation");
 }
 
@@ -25,6 +26,22 @@ fn convert_symlink_target_null_byte_invalid() {
             target: "foo\0".into(),
         })),
     }
-    .into_name_and_node()
+    .try_into_name_and_node()
     .expect_err("must fail validation");
 }
+
+/// Create a node with a name, and ensure our ano
+#[test]
+fn convert_anonymous_with_name_fail() {
+    assert_eq!(
+        DirectoryError::NameInAnonymousNode,
+        Node {
+            node: Some(node::Node::Symlink(SymlinkNode {
+                name: "foo".into(),
+                target: "somewhereelse".into(),
+            })),
+        }
+        .try_into_anonymous_node()
+        .expect_err("must fail")
+    )
+}