fix(python): make LanceDBClientError pickleable (#3470)

## Summary

- Add `__reduce__` methods to `LanceDBClientError` and `RetryError` so
that instances can be pickled and unpickled correctly
- `HttpError` inherits the fix from `LanceDBClientError` since it has no
additional `__init__` parameters
- Add tests verifying pickle roundtrip for all three exception classes

Fixes #3447

## Test plan

- [x] Verified pickle roundtrip for `LanceDBClientError` with and
without `status_code`
- [x] Verified pickle roundtrip for `HttpError` (subclass, no extra init
params)
- [x] Verified pickle roundtrip for `RetryError` (subclass with many
extra params)
- [ ] CI tests pass

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Will Jones <willjones127@gmail.com>

python/python/lancedb/remote/errors.py

@@ -27,6 +27,9 @@ class LanceDBClientError(RuntimeError):
         self.request_id = request_id
         self.status_code = status_code
 
+    def __reduce__(self) -> tuple[type, tuple]:
+        return (self.__class__, (str(self), self.request_id, self.status_code))
+
 
 class HttpError(LanceDBClientError):
     """An error that occurred during an HTTP request.
@@ -101,3 +104,19 @@ class RetryError(LanceDBClientError):
         self.max_request_failures = max_request_failures
         self.max_connect_failures = max_connect_failures
         self.max_read_failures = max_read_failures
+
+    def __reduce__(self) -> tuple[type, tuple]:
+        return (
+            self.__class__,
+            (
+                str(self),
+                self.request_id,
+                self.request_failures,
+                self.connect_failures,
+                self.read_failures,
+                self.max_request_failures,
+                self.max_connect_failures,
+                self.max_read_failures,
+                self.status_code,
+            ),
+        )

python/python/tests/test_errors.py

@@ -0,0 +1,56 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright The LanceDB Authors
+
+import pickle
+
+from lancedb.remote.errors import HttpError, LanceDBClientError, RetryError
+
+
+def test_pickle_lancedb_client_error():
+    err = LanceDBClientError("something went wrong", "req-123", 400)
+    restored = pickle.loads(pickle.dumps(err))
+    assert str(restored) == "something went wrong"
+    assert restored.request_id == "req-123"
+    assert restored.status_code == 400
+
+
+def test_pickle_lancedb_client_error_no_status_code():
+    err = LanceDBClientError("fail", "req-456")
+    restored = pickle.loads(pickle.dumps(err))
+    assert str(restored) == "fail"
+    assert restored.request_id == "req-456"
+    assert restored.status_code is None
+
+
+def test_pickle_http_error():
+    err = HttpError("not found", "req-789", 404)
+    restored = pickle.loads(pickle.dumps(err))
+    assert isinstance(restored, HttpError)
+    assert str(restored) == "not found"
+    assert restored.request_id == "req-789"
+    assert restored.status_code == 404
+
+
+def test_pickle_retry_error():
+    err = RetryError(
+        "max retries exceeded",
+        "req-abc",
+        request_failures=3,
+        connect_failures=1,
+        read_failures=2,
+        max_request_failures=5,
+        max_connect_failures=3,
+        max_read_failures=3,
+        status_code=503,
+    )
+    restored = pickle.loads(pickle.dumps(err))
+    assert isinstance(restored, RetryError)
+    assert str(restored) == "max retries exceeded"
+    assert restored.request_id == "req-abc"
+    assert restored.request_failures == 3
+    assert restored.connect_failures == 1
+    assert restored.read_failures == 2
+    assert restored.max_request_failures == 5
+    assert restored.max_connect_failures == 3
+    assert restored.max_read_failures == 3
+    assert restored.status_code == 503