Add README for MLServer model car tests

Author: Jooho

tests/model_serving/model_runtime/mlserver/model_car/README.md

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+# MLServer Model Car (OCI Image) Tests
+
+End-to-end tests for MLServer inference using model car (OCI image-based) deployments.
+Models are packaged as OCI container images and deployed via KServe InferenceService with `storageUri: oci://...`.
+
+## Supported Model Formats
+
+- sklearn
+- xgboost
+- lightgbm
+- onnx
+
+## OCI Model Images
+
+The OCI model images used in these tests are built from:
+https://github.com/Jooho/oci-model-images
+
+If the version of a supported framework (xgboost, lightgbm, sklearn, onnx) changes in MLServer,
+the model images must be rebuilt and pushed from that repository.
+
+The framework versions used by MLServer can be found at:
+https://github.com/red-hat-data-services/MLServer/blob/main/requirements/requirements-cpu.txt#L261
+
+For e2e testing, images should be tagged with the `-e2e` suffix to pin stable versions.
+
+## Running Tests
+
+Run all model car tests:
+
+```bash
+OC_BINARY_PATH=/usr/local/bin/oc uv run pytest tests/model_serving/model_runtime/mlserver/model_car
+```
+
+Run a specific model format (e.g., onnx only):
+
+```bash
+OC_BINARY_PATH=/usr/local/bin/oc uv run pytest tests/model_serving/model_runtime/mlserver/model_car -k "onnx"
+```
+
+## Updating Snapshots
+
+If model responses change and snapshots need to be updated, add the `--snapshot-update` flag:
+
+```bash
+OC_BINARY_PATH=/usr/local/bin/oc uv run pytest tests/model_serving/model_runtime/mlserver/model_car --snapshot-update
+```