Add ut for tf_transformers (#327)

## Summary

Add ut for tf_transformers

## Details

add ut for tf_transformers from 31% to 100%

<img width="1784" height="1428" alt="123667c9d71dcbdc198ebe2b84ba75a6"
src="https://github.com/user-attachments/assets/d2626a5f-38d3-4fdc-93b1-550bd4d95089"
/>
<img width="2398" height="934" alt="aa0509b6f9f134509a0d762f2a218fe4"
src="https://github.com/user-attachments/assets/29abc522-70e6-484b-87a4-7cd8b52a3cf7"
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## Test Plan

pytest -v tests/unit/utils/test_hf_transformers.py
<img width="1980" height="528" alt="0bd1b89ca91755096c162a4c82c11faa"
src="https://github.com/user-attachments/assets/f3df55a3-efcc-468f-987a-f4105c36060f"
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pytest --cov=guidellm tests/unit/utils
<img width="1916" height="540" alt="b975004df7a4a63e8272a43d00ca057f"
src="https://github.com/user-attachments/assets/db6cbffa-54bc-44a3-aa88-889dc6420207"
/>
<img width="2398" height="934" alt="aa0509b6f9f134509a0d762f2a218fe4"
src="https://github.com/user-attachments/assets/75a50ec6-c99f-4560-bf9d-8b2bcc6eb224"
/>


## Related Issues


- Resolves # none

---

- [x] "I certify that all code in this PR is my own, except as noted
below."

## Use of AI

- [x] Includes AI-assisted code completion
- [ ] Includes code generated by an AI application
- [ ] Includes AI-generated tests (NOTE: AI written tests should have a
docstring that includes `## WRITTEN BY AI ##`)

Author: sjmonson

tests/unit/utils/test_hf_transformers.py

@@ -0,0 +1,40 @@
+import pytest
+import transformers
+from transformers import PreTrainedTokenizerBase
+
+from guidellm.utils.hf_transformers import check_load_processor
+
+
+class DummyTokenizer(PreTrainedTokenizerBase):
+    pass
+
+
+def test_processor_is_none():
+    with pytest.raises(ValueError, match="Processor/Tokenizer is required for test."):
+        check_load_processor(None, None, "test")
+
+
+def test_processor_not_isinstance():
+    with pytest.raises(ValueError, match="Invalid processor/Tokenizer for test."):
+        check_load_processor(123, None, "test")  # type: ignore
+
+
+def test_processor_load_by_path(monkeypatch, tmp_path):
+    monkeypatch.setattr(
+        transformers.AutoTokenizer,
+        "from_pretrained",
+        lambda *args, **kwargs: DummyTokenizer(),
+    )
+    tokenizer = check_load_processor(tmp_path, None, "test")
+    assert isinstance(tokenizer, PreTrainedTokenizerBase)
+
+
+def test_processor_load_error(monkeypatch):
+    def raise_error(*args, **kwargs):
+        raise RuntimeError("test error")
+
+    monkeypatch.setattr("transformers.AutoTokenizer.from_pretrained", raise_error)
+    with pytest.raises(
+        ValueError, match="Failed to load processor/Tokenizer for test."
+    ):
+        check_load_processor("gpt2", None, "test")