test(python): cover VoyageAI text source routing (#3872)

## Summary

- add fast regression coverage for VoyageAI `voyage-3` source embeddings
- verify table text uses `client.embed` and never
`client.multimodal_embed`

## Root cause

The original VoyageAI source-embedding path treated table source values
as images and always invoked the multimodal API. Production routing was
corrected by later merged changes, but the table regression was covered
only by API-gated slow tests. This test locks the corrected text routing
into the regular unit suite.

## Validation

- `cd python && uv run --extra tests pytest
python/tests/test_voyageai_embeddings.py -q`
- `uv run --project python --extra tests --extra dev ruff format --check
python/python/tests/test_voyageai_embeddings.py`
- `uv run --project python --extra tests --extra dev ruff check .`

Fixes #2059

<!-- lance-gatekeeper-fix:v1 agent=49b9e2daeed95a78ce827e2bf90abda0
generation=1 -->

Co-authored-by: Gatefixer <313497061+lancedb-gatefixer[bot]@users.noreply.github.com>

python/python/tests/test_voyageai_embeddings.py

@@ -75,6 +75,22 @@ class TestVoyageAIModelRegistration:
         with pytest.raises(ValueError, match="not supported"):
             func.ndims()
 
+    def test_voyage3_source_embeddings_use_text_api(self, mock_voyageai_client):
+        """Regression test for text table data being sent to the multimodal API."""
+        mock_voyageai_client.tokenize.return_value = [["hello", "world"]]
+        mock_voyageai_client.embed.return_value.embeddings = [[0.1] * 1024]
+
+        registry = get_registry()
+        func = registry.get("voyageai").create(name="voyage-3")
+
+        embeddings = func.compute_source_embeddings("hello world")
+
+        assert embeddings == [[0.1] * 1024]
+        mock_voyageai_client.embed.assert_called_once_with(
+            texts=["hello world"], model="voyage-3", input_type="document"
+        )
+        mock_voyageai_client.multimodal_embed.assert_not_called()
+
     @pytest.mark.parametrize(
         "model_name",
         [