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.

---

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!)

---

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

libs/partners/openrouter/LICENSE

@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2025 LangChain, Inc.
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.