feat(openai): prevent silent streaming hangs in ChatOpenAI (#36949)

> [!IMPORTANT]
> **Behavior change on upgrade — minor bump (`1.1.16` → `1.2.0`).**
>
> Streaming calls now raise `StreamChunkTimeoutError` (a `TimeoutError`
subclass — existing `except TimeoutError:` / `except
asyncio.TimeoutError:` handlers catch it) after 120s of content silence
instead of hanging forever. Opt out with `stream_chunk_timeout=None` or
`LANGCHAIN_OPENAI_STREAM_CHUNK_TIMEOUT_S=0`.
>
> Kernel-level TCP keepalive / `TCP_USER_TIMEOUT` are applied via a
custom `httpx` transport. `httpx` disables its env-proxy auto-detection
(`HTTP_PROXY` / `HTTPS_PROXY` / `ALL_PROXY` / `NO_PROXY` and
macOS/Windows system proxy) whenever a transport is supplied, so to
avoid silently breaking enterprise proxy users, `ChatOpenAI` now detects
the "proxy-env-shadow" shape at construction and **skips the custom
transport entirely** when **all** of these hold:
>
> - `http_socket_options` left at default (`None`)
> - No `http_client` or `http_async_client` supplied
> - No `openai_proxy` supplied
> - A proxy env var / system proxy is visible to httpx
>
> On that shape the instance falls back to pre-PR behavior and env-proxy
auto-detection still applies. A one-time `INFO` records the bypass.
>
> Users who explicitly set `http_socket_options=[...]` alongside an env
proxy still get the shadowed behavior with a one-time `WARNING` log —
they opted in. Full opt-outs below.

---

Streaming chat completions can hang forever when the underlying TCP
connection silently dies mid-stream (idle NAT/LB timeouts, sandboxed
runtimes killing long-lived connections, peer gone without a FIN or
RST). httpx's read timeout doesn't help here because it's reset by any
bytes arriving on the socket, including OpenAI's SSE keepalive comments,
so a stream that's quiet on content but still producing keepalives looks
alive forever.

This PR adds two knobs to `ChatOpenAI`, both on by default with
opt-outs:

- `stream_chunk_timeout` (default 120s): wraps the async streaming
iterator in `asyncio.wait_for` per chunk. Measures the gap between
*parsed* SSE chunks, so keepalives don't reset it. Fires on genuine
content silence and raises `StreamChunkTimeoutError` — a `TimeoutError`
subclass carrying `timeout_s`, `model_name`, and `chunks_received` as
structured attributes (mirrored in the WARNING log's `extra=`) for
alerting without message-regex. Override with the kwarg or
`LANGCHAIN_OPENAI_STREAM_CHUNK_TIMEOUT_S`.
- `http_socket_options`: applies `SO_KEEPALIVE` + `TCP_KEEPIDLE` /
`TCP_KEEPINTVL` / `TCP_KEEPCNT` + `TCP_USER_TIMEOUT` on Linux (macOS
equivalents where available). On platforms missing some options, they're
dropped silently and the remaining set still does useful work.

Pool limits are set explicitly on the custom transport to mirror the
`openai` SDK — without that, passing `transport=` to `httpx.AsyncClient`
silently shrinks the connection pool.

## Behavior change

The default-shape proxy-env bypass (above) covers the common enterprise
case. Beyond that:

- Connections that would previously have hung forever will now error out
via `StreamChunkTimeoutError`.
- Users who explicitly opt into `http_socket_options` while also relying
on env proxies will see a one-time `WARNING` and lose env-proxy
auto-detection — the custom transport shadows it. This is the original
shipped behavior, retained for anyone who *wants* socket tuning on top
of an env-proxied setup.

Full opt-outs:

- `stream_chunk_timeout=None` or
`LANGCHAIN_OPENAI_STREAM_CHUNK_TIMEOUT_S=0`
- `http_socket_options=()` or `LANGCHAIN_OPENAI_TCP_KEEPALIVE=0`
- Supply your own `http_client` **and** `http_async_client`.
`http_socket_options` is applied per side: passing only one still leaves
the other side's default builder getting socket options. Supply both (or
combine with `http_socket_options=()`) to take full control.

Unparseable or negative values for the `LANGCHAIN_OPENAI_*` env vars
fall back to the default with a `WARNING` log rather than silently being
accepted, so a misconfigured environment still boots but the fallback is
discoverable.

---------

Co-authored-by: Mason Daugherty <github@mdrxy.com>
Co-authored-by: Mason Daugherty <mason@langchain.dev>

libs/partners/openai/langchain_openai/__init__.py

@@ -1,6 +1,7 @@
 """Module for OpenAI integrations."""
 
 from langchain_openai.chat_models import AzureChatOpenAI, ChatOpenAI
+from langchain_openai.chat_models._client_utils import StreamChunkTimeoutError
 from langchain_openai.embeddings import AzureOpenAIEmbeddings, OpenAIEmbeddings
 from langchain_openai.llms import AzureOpenAI, OpenAI
 from langchain_openai.tools import custom_tool
@@ -12,5 +13,6 @@ __all__ = [
     "ChatOpenAI",
     "OpenAI",
     "OpenAIEmbeddings",
+    "StreamChunkTimeoutError",
     "custom_tool",
 ]