Checkpoint save fails with ENOENT during OCDBT atomic rename on TPU v5litepod

## Environment

- **TPU:** v5litepod-32 (32 chips, 8 hosts)
- **JAX:** 0.9.0
- **jaxlib:** 0.9.0
- **orbax-checkpoint:** 0.11.32
- **Python:** 3.12
- **OS:** Ubuntu 22.04 (tpu-ubuntu2204-base)

## Problem

When converting a HuggingFace Llama 8B checkpoint to MaxText format using `llama_or_mistral_ckpt.py`, the checkpoint save fails silently. The layer conversion completes successfully (32/32 layers), but the Orbax save operation fails during atomic file rename.

## Error Message

```
ValueError: NOT_FOUND: Error writing "params.params.decoder.decoder_norm.scale/c/0" in OCDBT database at local file "/home/user/maxtext-checkpoints/r1-distill-llama-8b/0.orbax-checkpoint-tmp/items.orbax-checkpoint-tmp/ocdbt.process_0/": Failed to rename fd: 14 "/home/user/maxtext-checkpoints/r1-distill-llama-8b/0.orbax-checkpoint-tmp/items.orbax-checkpoint-tmp/ocdbt.process_0/d/8f39efd1e47e6a0ada381d99719b652c.__lock" to: "/home/user/maxtext-checkpoints/r1-distill-llama-8b/0.orbax-checkpoint-tmp/items.orbax-checkpoint-tmp/ocdbt.process_0/d/8f39efd1e47e6a0ada381d99719b652c" [OS error 2: ENOENT No such file or directory] [source locations='tensorstore/internal/os/file_util_posix.cc:406\ntensorstore/kvstore/file/file_key_value_store.cc:662\ntensorstore/kvstore/transaction.cc:133']
```

## Full Stack Trace

```python
File "/home/user/venv312/lib/python3.12/site-packages/orbax/checkpoint/_src/futures/future.py", line 309, in _target_setting_result
self._result = target()
File "/home/user/venv312/lib/python3.12/site-packages/orbax/checkpoint/_src/futures/future.py", line 426, in
target=lambda: asyncio_utils.run_sync(coro),
File "/home/user/venv312/lib/python3.12/site-packages/orbax/checkpoint/_src/asyncio_utils.py", line 36, in run_sync
return asyncio.run(coro)
File "/home/user/venv312/lib/python3.12/site-packages/orbax/checkpoint/_src/serialization/jax_array_handlers.py", line 373, in _serialize_without_dispatcher
await async_serialize_replica_slices_batch(
File "/home/user/venv312/lib/python3.12/site-packages/orbax/checkpoint/_src/serialization/jax_array_handlers.py", line 618, in _async_serialize_replica_slices
await asyncio.gather(*write_coros)
File "/home/user/venv312/lib/python3.12/site-packages/orbax/checkpoint/_src/serialization/serialization.py", line 265, in async_serialize_from_host
await asyncio.gather(*write_coros)
File "/home/user/venv312/lib/python3.12/site-packages/orbax/checkpoint/_src/serialization/serialization.py", line 253, in write_fragment
await t[fragment.index].write(
ValueError: NOT_FOUND: Error writing "params.params.decoder.decoder_norm.scale/c/0" in OCDBT database...
```

## Steps to Reproduce

1. Create a v5litepod-32 TPU
2. Install MaxText: `pip install -e .` from https://github.com/AI-Hypercomputer/maxtext
3. Download a HuggingFace Llama model (e.g., deepseek-ai/DeepSeek-R1-Distill-Llama-8B)
4. Run the checkpoint conversion:

```bash
JAX_PLATFORMS=cpu python3 llama_or_mistral_ckpt.py \
--base-model-path=/path/to/hf-model \
--maxtext-model-path=/path/to/output \
--model-size=llama3-8b \
--huggingface-checkpoint=True
```

## Observed Behavior

1. Layer conversion completes: `layers: 100%|██████████| 32/32`
2. Sharding conversion completes
3. Orbax save starts but fails during atomic rename
4. Checkpoint directory contains ~11GB of data but is incomplete (missing tree structure metadata)
5. Subsequent attempts to load the checkpoint fail with "No structure could be identified"

## What We Tried

- Saving to local disk instead of GCS - same issue
- Using `JAX_PLATFORMS=cpu` - same issue
- Disabling OCDBT/Zarr3 with legacy format flags - same issue (save appears to succeed but 0 bytes written)
- Different JAX versions (0.5.0, 0.8.1, 0.9.0) - same issue

## Analysis

The error appears to be a race condition or file descriptor issue in the tensorstore OCDBT layer:
1. Orbax creates a `.lock` file during write
2. Tries to atomically rename `.lock` → final file
3. Rename fails with ENOENT despite the file existing moments before

This may be related to async coordination between multiple write operations.

## Expected Behavior

Checkpoint conversion should complete successfully and produce a loadable MaxText checkpoint.

## Impact

This completely blocks the ability to convert HuggingFace models to MaxText format on multi-host TPUs, which is a primary use case for the MaxText + JetStream serving stack.