feat(openrouter): add `langchain-openrouter` provider package (#35211)

Add a first-party `langchain-openrouter` partner package
(`ChatOpenRouter`) that wraps the official `openrouter` Python SDK,
providing native support for OpenRouter-specific features that
`ChatOpenAI` intentionally does not handle.

Also adds scope-clarifying docstrings to `ChatOpenAI` / `BaseChatOpenAI`
warning users away from using `base_url` overrides with third-party
providers.

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Closes #31325
Closes #32967
Closes #32977
Closes #32981
Closes #33643
Closes #33757
Closes #34056
Closes #34797
Closes #34962

Supersedes #33902, #34867 (thank you @elonfeng and @okamototk for your
initial work on this!)

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Bugs with upstream sdk:
- https://github.com/OpenRouterTeam/python-sdk/issues/38
- https://github.com/OpenRouterTeam/python-sdk/issues/51
- https://github.com/OpenRouterTeam/python-sdk/issues/52