feat: support Blob v2 UDF signatures (#4091)

Make Function authoring and declaration planning treat Blob v2 as a
scalar semantic type while preserving exact Blob metadata in binding
schemas.

Covers scalar Blob outputs, expanded named-struct outputs, and
whole-result structs with Blob children.

python/python/tests/test_first_class_function_slice2.py

@@ -578,6 +578,124 @@ def test_explicit_arrow_schema_is_deterministic():
     assert signature.output.nullable is False
 
 
+def test_blob_fields_use_the_scalar_function_semantic_type():
+    @udf(
+        input_schema=pa.schema([lancedb.blob("image", nullable=False)]),
+        output_schema=lancedb.blob("result", nullable=False),
+    )
+    def copy_blob(image):
+        return image
+
+    signature = copy_blob.registration_request.signature
+    assert signature.inputs[0].arrow_type == "blob_v2"
+    assert signature.output.kind == "scalar"
+    assert signature.output.arrow_type == "blob_v2"
+
+
+def test_named_struct_function_can_include_a_blob_result_field():
+    @udf(
+        input_schema=pa.schema([lancedb.blob("image", nullable=False)]),
+        output_schema=pa.schema(
+            [
+                lancedb.blob("thumbnail", nullable=False),
+                pa.field("width", pa.int32(), nullable=False),
+            ]
+        ),
+    )
+    def inspect_blob(image):
+        return {"thumbnail": image, "width": 1}
+
+    output = inspect_blob.registration_request.signature.output
+    assert output.kind == "named_struct"
+    assert [(field.name, field.arrow_type) for field in output.fields] == [
+        ("thumbnail", "blob_v2"),
+        ("width", "int32"),
+    ]
+
+
+def test_metadata_marked_blob_field_uses_the_semantic_type():
+    extension = lancedb.blob("image", nullable=False).type
+    storage = (
+        extension.storage_type if isinstance(extension, pa.ExtensionType) else extension
+    )
+    metadata_blob = pa.field(
+        "image",
+        storage,
+        nullable=False,
+        metadata={"ARROW:extension:name": "lance.blob.v2"},
+    )
+
+    @udf(
+        input_schema=pa.schema([metadata_blob]),
+        output_schema=pa.field("size", pa.int64(), nullable=False),
+    )
+    def blob_size(image):
+        return len(image)
+
+    assert blob_size.registration_request.signature.inputs[0].arrow_type == "blob_v2"
+
+
+def test_blob_marker_rejects_invalid_storage_layout():
+    malformed = pa.field(
+        "image",
+        pa.int64(),
+        nullable=False,
+        metadata={"ARROW:extension:name": "lance.blob.v2"},
+    )
+
+    with pytest.raises(TypeError, match="requires a supported Blob storage layout"):
+
+        @udf(
+            input_schema=pa.schema([malformed]),
+            output_schema=pa.field("size", pa.int64(), nullable=False),
+        )
+        def blob_size(image):
+            return len(image)
+
+
+def test_nested_blob_signature_field_has_a_clear_error():
+    nested = pa.field(
+        "value",
+        pa.struct([lancedb.blob("image", nullable=False)]),
+        nullable=False,
+    )
+    with pytest.raises(TypeError, match="nested Blob v2 fields are not supported"):
+
+        @udf(
+            input_schema=pa.schema([nested]),
+            output_schema=pa.field("size", pa.int64(), nullable=False),
+        )
+        def blob_size(value):
+            return len(value["image"])
+
+
+def test_nested_non_blob_extension_is_not_silently_unwrapped():
+    class TestExtension(pa.ExtensionType):
+        def __init__(self):
+            super().__init__(pa.int64(), "test.function.extension")
+
+        def __arrow_ext_serialize__(self):
+            return b""
+
+        @classmethod
+        def __arrow_ext_deserialize__(cls, storage_type, serialized):
+            return cls()
+
+    nested = pa.field(
+        "value",
+        pa.struct([pa.field("extended", TestExtension(), nullable=False)]),
+        nullable=False,
+    )
+    with pytest.raises(TypeError, match="unsupported Arrow type"):
+
+        @udf(
+            input_schema=pa.schema([nested]),
+            output_schema=pa.field("result", pa.int64(), nullable=False),
+        )
+        def extension_value(value):
+            return value["extended"]
+
+
 def test_nested_struct_output_uses_canonical_exact_json():
     token = pa.struct(
         [