Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@crates/lib/src/core/db/merkle_node/file_backend.rs`:
- Around line 63-70: get_children currently calls MerkleNodeDB::open_read_only
directly and will propagate an IO error for missing hashes; make its behavior
consistent with get_node by first checking self.exists(hash) and returning
Ok(vec![]) (or Ok(None) if you prefer Option semantics and update signature/call
sites) when the hash doesn't exist, otherwise proceed to open_read_only and
map(); reference get_children, get_node, exists(), and
MerkleNodeDB::open_read_only when making the change.

---

Nitpick comments:
In `@crates/lib/src/core/db/merkle_node/file_backend.rs`:
- Around line 150-156: The Drop impl for FileNodeSession currently calls
self.idempotent_finish().expect(...), which will panic on I/O errors during
drop; change it to call idempotent_finish() and handle the Result without
panicking (e.g., match or if let Err(e) = ...) and log the error instead of
calling expect so drop never unwinds; reference the Drop for FileNodeSession,
the drop() method, idempotent_finish(), and the explicit finish() API (callers
who need to observe errors should use finish()) when locating the change.

Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@crates/lib/src/core/db/merkle_node/file_backend.rs`:
- Around line 47-55: The code returns db.node_id which is left as zero by
MerkleNodeDB::open_read_only; change the construction of MerkleNodeRecord in
this block to pass the requested hash (the local variable `hash`) instead of
`db.node_id` so the returned record reports the actual requested node id; update
the call to MerkleNodeRecord::new(...) to use `hash` as the first argument.
- Around line 140-155: The current idempotent_finish marks
FileNodeSession::finished before calling MerkleNodeDB::close which suppresses
retries on failure and the Drop impl panics via expect; change idempotent_finish
so it only sets self.finished = true after MerkleNodeDB::close(&mut self.db)
returns Ok (i.e., call close first and on success set finished), and modify the
Drop implementation for FileNodeSession to not panic—call
self.idempotent_finish() and ignore or log errors (e.g., .ok()) instead of using
expect so drop never aborts during unwinding.
- Around line 79-80: The implementation of MerkleWriter::begin uses a mismatched
self lifetime; change the signature in file_backend.rs from fn begin<'a>(&'a
self) -> Result<Box<dyn MerkleWriteSession + 'a>, OxenError> to fn
begin<'a>(&self) -> Result<Box<dyn MerkleWriteSession + 'a>, OxenError> so it
exactly matches the MerkleWriter trait; keep the body that returns
Box::new(FileWriteSession { repo: self.repo }) and ensure FileWriteSession and
the returned Box implement MerkleWriteSession with the correct lifetimes.

Verify each finding against the current code and only fix it if needed.

Nitpick comments:
In `@crates/lib/src/model/merkle_tree/node.rs`:
- Around line 76-92: Add a table-driven round-trip unit test that for each
concrete node type (CommitNode, DirNode, FileNode, VNode, FileChunkNode)
constructs a representative instance, serializes it to msgpack bytes (using the
same serializer used elsewhere), obtains its dtype via the instance's
node_type() or equivalent, then calls
EMerkleTreeNode::from_type_and_bytes(dtype, &bytes) and asserts the returned
EMerkleTreeNode variant matches and equals the original (or its contained
struct). Ensure the test covers all variants of the EMerkleTreeNode enum and
fails if any dtype→decoder mapping in from_type_and_bytes is incorrect.