fix(python): honor explicit OpenAI reranker API keys (#4515)

Fixes #4514

## Summary
Honor an explicitly supplied `OpenaiReranker(api_key=...)` before
falling back to `OPENAI_API_KEY`, matching the constructor
documentation. Previously the environment key always won when both were
set, so an instance configured for a different account still used the
process-wide account.

## Tests
Added four cases using real OpenAI client construction without sending
requests: explicit/environment conflict, explicit key alone, environment
fallback, and missing credentials. The conflict case fails before the
change; all four pass afterward. Clients are closed after each
assertion.

- Targeted source-module tests: 4 passed.
- Ruff check and format check: passed for both changed files.
- Full native source build was attempted but failed because the
environment has no Rust toolchain; bootstrap then attempted a cache path
outside the writable workspace. The targeted tests load the current
Python reranker module against installed dependencies and the released
LanceDB package; no database/native behavior is exercised. Full
native/integration suite not run.

This change and its tests were prepared with an AI coding agent. The
diff was reviewed and the failure/pass behavior verified locally.

Co-authored-by: Xuanwo <github@xuanwo.io>

python/python/lancedb/rerankers/openai.py

@@ -131,4 +131,4 @@ class OpenaiReranker(Reranker):
                 "OPENAI_API_KEY not set. Either set it in your environment or \
                 pass it as `api_key` argument to the CohereReranker."
             )
-        return openai.OpenAI(api_key=os.environ.get("OPENAI_API_KEY") or self.api_key)
+        return openai.OpenAI(api_key=self.api_key or os.environ.get("OPENAI_API_KEY"))

python/python/tests/test_openai_reranker_credentials.py

@@ -0,0 +1,38 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright The LanceDB Authors
+
+import pytest
+
+from lancedb.rerankers import OpenaiReranker
+
+
+@pytest.mark.parametrize(
+    "explicit_key, environment_key, expected_key",
+    [
+        ("explicit-test-key", "environment-test-key", "explicit-test-key"),
+        ("explicit-test-key", None, "explicit-test-key"),
+        (None, "environment-test-key", "environment-test-key"),
+    ],
+)
+def test_openai_reranker_api_key_precedence(
+    monkeypatch, explicit_key, environment_key, expected_key
+):
+    pytest.importorskip("openai")
+    if environment_key is None:
+        monkeypatch.delenv("OPENAI_API_KEY", raising=False)
+    else:
+        monkeypatch.setenv("OPENAI_API_KEY", environment_key)
+
+    reranker = OpenaiReranker(api_key=explicit_key)
+    client = reranker._client
+    try:
+        assert client.api_key == expected_key
+    finally:
+        client.close()
+
+
+def test_openai_reranker_requires_api_key(monkeypatch):
+    pytest.importorskip("openai")
+    monkeypatch.delenv("OPENAI_API_KEY", raising=False)
+    with pytest.raises(ValueError, match="OPENAI_API_KEY not set"):
+        OpenaiReranker()._client