docs(openai): document ChatOpenAI endpoint resolution precedence

[Mohan Ram (mohanram-dev)]

Description

Fixes #11635

This PR documents the base URL endpoint resolution order in @langchain/openai and adds unit test coverage to ensure parity with the Python counterpart (langchain-openai).

In @langchain/openai, ChatOpenAI resolves its endpoint according to the following precedence:

1. configuration.baseURL (explicit client option)
2. baseUrl (serialized/loaded compatibility field)
3. OPENAI_API_BASE environment variable (read by LangChain — takes precedence over OPENAI_BASE_URL)
4. OPENAI_BASE_URL environment variable (read by the underlying OpenAI SDK)
5. Default OpenAI API base URL (https://api.openai.com/v1)

Because the underlying openai SDK reads only OPENAI_BASE_URL and ignores OPENAI_API_BASE, a leftover OPENAI_API_BASE in the environment can silently override a newly configured OPENAI_BASE_URL. This PR adds explicit JSDoc documentation and regression test assertions to make this priority explicit and clear for developers.

Changes Made

• Documented resolution order on BaseChatOpenAIFields.configuration in libs/providers/langchain-openai/src/chat_models/base.ts.
• Updated ChatOpenAI class JSDoc in libs/providers/langchain-openai/src/chat_models/index.ts.
• Added unit tests in libs/providers/langchain-openai/src/chat_models/tests/index.test.ts verifying OPENAI_API_BASE takes precedence over OPENAI_BASE_URL, and configuration.baseURL takes precedence over environment variables.

Validation

• pnpm --filter @langchain/core build passed.
• pnpm --filter @langchain/openai build passed.
• pnpm --filter @langchain/openai test src/chat_models/tests/index.test.ts passed (54/54 tests).
• pnpm format:check and pnpm lint passed with 0 errors.

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