feat(python): support Enum types in Pydantic to Arrow schema conversion (#3232)

## Summary

Fixes #1846.

Python `Enum` fields raised `TypeError: Converting Pydantic type to
Arrow Type: unsupported type <enum 'SomethingTypes'>` when converting a
Pydantic model to an Arrow schema.

The fix adds Enum detection in `_pydantic_type_to_arrow_type`. When an
Enum subclass is encountered, the value type of its members is inspected
and mapped to the appropriate Arrow type:

- `str`-valued enums (e.g. `class Status(str, Enum)`) → `pa.utf8()`
- `int`-valued enums (e.g. `class Priority(int, Enum)`) → `pa.int64()`
- Other homogeneous value types → the Arrow type for that Python type
- Mixed-value or empty enums → `pa.utf8()` (safe fallback)

This covers the common `(str, Enum)` and `(int, Enum)` mixin patterns
used in practice.

## Changes

- `python/python/lancedb/pydantic.py`: add Enum branch in
`_pydantic_type_to_arrow_type`
- `python/python/tests/test_pydantic.py`: add `test_enum_types` covering
`str`, `int`, and `Optional` Enum fields

## Note on #2395

PR #2395 handles `StrEnum` (Python 3.11+) specifically, using a
dictionary-encoded type. This PR handles the broader `(str, Enum)` /
`(int, Enum)` mixin pattern that works across all Python versions and
stores values as their natural Arrow type.

AI assistance was used in developing this fix.

python/python/tests/test_pydantic.py

@@ -3,6 +3,7 @@
 
 import json
 from datetime import date, datetime
+from enum import Enum
 from typing import List, Optional, Tuple
 
 import pyarrow as pa
@@ -673,3 +674,29 @@ async def test_aliases_in_lance_model_async(mem_db_async):
     assert hasattr(model, "name")
     assert hasattr(model, "distance")
     assert model.distance < 0.01
+
+
+def test_enum_types():
+    """Enum fields should map to the Arrow type of their value (issue #1846)."""
+
+    class StrStatus(str, Enum):
+        PENDING = "pending"
+        RUNNING = "running"
+        DONE = "done"
+
+    class IntPriority(int, Enum):
+        LOW = 1
+        MEDIUM = 2
+        HIGH = 3
+
+    class TestModel(pydantic.BaseModel):
+        status: StrStatus
+        priority: IntPriority
+        opt_status: Optional[StrStatus] = None
+
+    schema = pydantic_to_schema(TestModel)
+
+    assert schema.field("status").type == pa.dictionary(pa.int32(), pa.utf8())
+    assert schema.field("priority").type == pa.int64()
+    assert schema.field("opt_status").type == pa.dictionary(pa.int32(), pa.utf8())
+    assert schema.field("opt_status").nullable