feat(standard-tests): forward LangSmith CI env vars to traces (#37645)

Scheduled integration runs set `LANGSMITH_TAGS` and `LANGSMITH_METADATA`
in `$GITHUB_ENV` (per #37615), but the LangSmith SDK does not read those
env vars natively, so the tags/metadata were silently dropped. A new
pytest plugin in `langchain-tests` bridges that gap by entering
`langsmith.run_helpers.tracing_context` for the duration of each
session.

libs/standard-tests/langchain_tests/_langsmith_plugin.py

@@ -0,0 +1,91 @@
+"""Pytest plugin that forwards CI-set LangSmith env vars into tracing context.
+
+The LangSmith SDK does not natively read `LANGSMITH_TAGS` / `LANGSMITH_METADATA`
+from the environment, so tags/metadata written to `$GITHUB_ENV` by CI workflows
+(e.g. `integration_tests.yml`) would otherwise be silently dropped. This plugin
+bridges that gap for the duration of the pytest session by entering
+`langsmith.run_helpers.tracing_context`.
+
+To avoid surprising developers who happen to export these vars locally, the
+plugin only activates when running under GitHub Actions (`GITHUB_ACTIONS=true`).
+
+Auto-discovered by pytest in any package that depends on `langchain-tests`
+(declared via the `pytest11` entry point in
+`libs/standard-tests/pyproject.toml`).
+"""
+
+from __future__ import annotations
+
+import contextlib
+import json
+import os
+import sys
+import warnings
+from typing import TYPE_CHECKING, Any
+
+import pytest
+from langsmith.run_helpers import tracing_context
+
+if TYPE_CHECKING:
+    from collections.abc import Iterator
+
+
+def _is_github_actions() -> bool:
+    return os.environ.get("GITHUB_ACTIONS") == "true"
+
+
+def _parse_tags(raw: str) -> list[str]:
+    return [tag.strip() for tag in raw.split(",") if tag.strip()]
+
+
+def _parse_metadata(raw: str) -> dict[str, Any] | None:
+    raw = raw.strip()
+    if not raw:
+        return None
+    try:
+        parsed = json.loads(raw)
+    except json.JSONDecodeError as exc:
+        _warn_loud(f"Ignoring LANGSMITH_METADATA: invalid JSON ({exc}).")
+        return None
+    if not isinstance(parsed, dict):
+        _warn_loud("Ignoring LANGSMITH_METADATA: expected a JSON object.")
+        return None
+    return parsed
+
+
+def _warn_loud(message: str) -> None:
+    """Emit a `UserWarning` and mirror it to stderr so CI logs surface it."""
+    warnings.warn(message, UserWarning, stacklevel=3)
+    sys.stderr.write(f"[langchain-tests] {message}\n")
+
+
+@contextlib.contextmanager
+def _langsmith_ci_cm() -> Iterator[None]:
+    """Yield with tracing context applied from current env vars.
+
+    No-op when neither env var is set or when not running on GitHub Actions.
+    """
+    if not _is_github_actions():
+        yield
+        return
+
+    tags = _parse_tags(os.environ.get("LANGSMITH_TAGS", ""))
+    metadata = _parse_metadata(os.environ.get("LANGSMITH_METADATA", ""))
+
+    if not tags and not metadata:
+        yield
+        return
+
+    keys = sorted(metadata or {})
+    sys.stderr.write(
+        f"[langchain-tests] langsmith CI context: tags={tags}, metadata_keys={keys}\n",
+    )
+    with tracing_context(tags=tags or None, metadata=metadata):
+        yield
+
+
+@pytest.fixture(scope="session", autouse=True)
+def _langsmith_ci_context() -> Iterator[None]:
+    """Apply `LANGSMITH_TAGS`/`LANGSMITH_METADATA` to traces in this session."""
+    with _langsmith_ci_cm():
+        yield