feat(openai): add openai unit test for streaming error in _generate

[Ali-Ismail-1]

Description:

• Summary: Adds a unit test to ensure _generate raises a ValueError when streaming=True and multiple prompts are provided.
• Motivation: Adds test coverage to address TODO in libs/partners/openai/langchain_openai/llms/base.py
• Changes: Added a unit test for _generate

---

Test Plan

run the test file with
make test TEST_FILE=tests/unit_tests/llms/test_base.py

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1 Skipped Deployment

Project	Deployment	Preview	Comments	Updated (UTC)
langchain	Ignored	Preview		Sep 30, 2025 10:28pm

[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #33134 will not alter performance

_{Comparing Ali-Ismail-1:feat/openai-llm-generate-unit-tests (3bab7cf) with master (aac6983)¹}

Summary

✅ 6 untouched
⏩ 15 skipped²

Footnotes

1. No successful run was found on master (a541b5b) during the generation of this report, so aac6983 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

2. 15 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩