feat(openrouter): session_id and trace fields (#37137)

Add first-class `session_id` and `trace` constructor fields on
`ChatOpenRouter`, plumbed into the request payload alongside the
existing `route` / `plugins` / `openrouter_provider` knobs. Both
correspond to the OpenRouter
[broadcast](https://openrouter.ai/docs/guides/features/broadcast/overview)
feature for grouping requests under one logical workflow and attaching
per-request observability metadata. Previously these were only reachable
by stuffing them through `model_kwargs` or `.bind()`, neither of which
is discoverable.

[Docs](https://github.com/langchain-ai/docs/pull/3840)

## Changes
- New `session_id: str | None` field with a
`from_env("OPENROUTER_SESSION_ID", default=None)` factory, so a process
can group all requests via env var without threading the value through
application code. Truthy-guarded in `_default_params` so an explicit or
env-sourced empty string is treated as unset.
- New `trace: dict[str, Any] | None` field for per-request trace
metadata (`trace_id`, `trace_name`, `span_name`, `generation_name`,
`parent_span_id`, plus arbitrary extras forwarded as custom metadata).
No env fallback — set per-call or on the constructor.
- Per-call override (`model.invoke(..., session_id=..., trace=...)`)
works through the existing `**kwargs` flow into `_generate` / `_stream`,
with the constructor value preserved across calls.
- Updated the "Key init args — client params" docstring table on
`ChatOpenRouter` to surface both fields.

libs/partners/openrouter/uv.lock

@@ -470,7 +470,7 @@ wheels = [
 
 [[package]]
 name = "langchain-tests"
-version = "1.1.6"
+version = "1.1.7"
 source = { editable = "../../standard-tests" }
 dependencies = [
     { name = "httpx" },