[Bug] Misleading "Unable to read the model" error when intermediate IR is truncated during INT4 export

### Environment

Verified on the latest tooling supported by optimum-intel `main`:

- `optimum-intel`: 1.27.0.dev0+190d59f (main HEAD, also released 1.27.0)
- `transformers`: 5.0.0 (latest within the `<5.1` constraint set by optimum-intel main)
- `openvino`: 2026.1.0
- `nncf`: 3.1.0
- `torch`: 2.11.0
- Python: 3.12.3, Linux x86_64

### Description

When `optimum-cli export openvino --weight-format int4 ...` runs against a
large model and the temp directory backing the FP16 intermediate IR fills up,
the FP16 write is silently truncated by `ENOSPC` and the subsequent NNCF
int4 reload of the IR fails. The error never mentions disk space.

The exact OpenVINO error depends on how much of the XML/BIN pair survived
the truncation. Repro'd on the latest stack against `OVBaseModel.load_model`:

| Truncation | Error |
|----------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| XML truncated to 0 bytes | `Unable to read the model: "..." Please check that model format: ".xml" is supported and the model is correct. Available frontends: onnx jax tflite paddle ir tf pytorch` |
| XML truncated mid-tag (e.g. 50%) | `Check 'res.status == pugi::status_ok' failed at src/frontends/ir/src/input_model.cpp:222: Error parsing element attribute at offset N` |
| XML almost intact (e.g. 99%) | `Check 'res.status == pugi::status_ok' failed at src/frontends/ir/src/input_model.cpp:222: Start-end tags mismatch at offset N` |
| BIN truncated (any non-zero loss)| `Check 'm_weights->size() >= offset + size' failed at src/core/xml_util/src/xml_deserialize_util.cpp:907: Incorrect weights in bin file!` |

In each case, the user has no signal that disk is the cause; they have to
inspect the temp directory by hand to discover that the XML/BIN pair was
truncated.

### Steps to reproduce

The realistic path is to run the actual export on a host with insufficient
free space at `TMPDIR`:

```bash
TMPDIR=/tmp \
optimum-cli export openvino \
--model meta-llama/Llama-3.1-70B-Instruct \
--weight-format int4 \
--task text-generation-with-past \
/path/to/output
```

A self-contained reproducer that does **not** require a 70 GB model is
attached at
[`upstream/04_optimum_intel_disk_full_error/reproducer.py`](https://github.com/labscommunity/rainier/blob/main/upstream/04_optimum_intel_disk_full_error/reproducer.py)
in our project repo: it exports a tiny model, truncates the resulting
`model.xml` (or `.bin`) to several levels, and calls
`OVBaseModel.load_model` to surface each error path.

### Expected behavior

Either (a) the export aborts earlier with a clear "not enough free space at
TMPDIR" message, or (b) the read failure includes the file sizes and free
disk space at the path so the user can diagnose immediately.

### Where this happens in the source

`optimum/commands/export/openvino.py:455-462` creates a `TemporaryDirectory()`
that honours `TMPDIR` and falls back to `/tmp`. There is no free-space
precheck.

The FP16 IR is then written by `main_export` →
`optimum/exporters/openvino/convert.py` (`_save_model` →
`openvino.save_model`). `_main_quantize` re-reads it via
`model_cls.from_pretrained(output, ...)`, which lands in
`OVBaseModel.load_model` at
`optimum/intel/openvino/modeling_base.py:369-376`
(line numbers against upstream `main` at HEAD `190d59f`):

```python
if isinstance(file_name, str):
file_name = Path(file_name)
model = (
core.read_model(file_name.resolve(), file_name.with_suffix(".bin").resolve())
if not file_name.suffix == ".onnx"
else convert_model(file_name)
)
```

There is no integrity check and no exception handling around `core.read_model`,
so any failure (truncation, permission, anything else) re-emits the verbatim
OpenVINO error.

### Proposed Solution

A minimal patch in `OVBaseModel.load_model` that catches the read failure and
adds disk-space + file-size context to the error:

```python
import shutil
...
if isinstance(file_name, str):
file_name = Path(file_name)
if file_name.suffix == ".onnx":
model = convert_model(file_name)
else:
xml_path = file_name.resolve()
bin_path = file_name.with_suffix(".bin").resolve()
try:
model = core.read_model(xml_path, bin_path)
except Exception as e:
xml_size = xml_path.stat().st_size if xml_path.is_file() else -1
bin_size = bin_path.stat().st_size if bin_path.is_file() else -1
try:
free_space = shutil.disk_usage(xml_path.parent).free
except OSError:
free_space = -1
raise RuntimeError(
f"OpenVINO failed to read {xml_path} "
f"(xml size={xml_size} bytes, bin size={bin_size} bytes, "
f"free space at parent dir={free_space} bytes). "
f"If file sizes look truncated, the intermediate IR write may have "
f"run out of disk space; check that TMPDIR (or the export output "
f"directory) has sufficient free space. "
f"Original error: {e}"
) from e
```

Happy to open a PR. The matching branch with this patch is at
[`t8/optimum-intel:improve-corrupt-ir-error-message`](https://github.com/t8/optimum-intel/tree/improve-corrupt-ir-error-message)
([commit 9f40fff](https://github.com/t8/optimum-intel/commit/9f40fff959b9380ba80076e97edbf17d8da26e6e),
+24 −5 in one file). Branch is rebased on main HEAD `190d59f`.

A follow-up improvement (separate PR if the first is welcome) would be to
add a free-space precheck in `optimum/commands/export/openvino.py` shortly
after `TemporaryDirectory()` is created: log
`shutil.disk_usage(temporary_directory.name).free` and warn when it is less
than ~2× the model's HF-config-implied FP16 size.