feat(langsmith)!: migrate wrapper to integration

[Abhijeet Prasad (AbhiPrasad)]

Move the LangSmith instrumentation into braintrust.integrations.langsmith and reduce the legacy wrapper to a setup_langsmith compatibility shim.

Add LangSmith to auto_instrument, add a dedicated nox session, and cover the patched traceable and evaluate surfaces with VCR-backed tests plus an auto-test script.

BREAKING CHANGE: remove the public LangSmith helper exports wrap_traceable, wrap_client, wrap_evaluate, and wrap_aevaluate from both braintrust.integrations.langsmith and braintrust.wrappers.langsmith_wrapper. The compatibility wrapper now exposes setup_langsmith only.

This breaking change is needed to ensure we can properly do the migration. We don't document wrap_traceable, wrap_client, wrap_evaluate, and wrap_aevaluate anywhere so we can easily do this.

[starfolkai]

Drop into this review session: sfk devbox connect pr-204-braintrust-sdk-python-5b02 --attach

Found 2 issues:

1. setup_langsmith(standalone=False) cannot override the BRAINTRUST_LANGSMITH_STANDALONE environment variable. The expression True if standalone else None on line 38 collapses False to None, which means "use the env var" rather than "explicitly disable standalone." The internal _set_langsmith_standalone_override function distinguishes None (defer to env var) from False (explicitly disabled), but the public API loses that distinction.

braintrust-sdk-python/py/src/braintrust/integrations/langsmith/__init__.py

Lines 37 to 38 in 8c9d4a6

	logger.info("LangSmith integration with Braintrust enabled")
	return LangSmithIntegration.setup(standalone=True if standalone else None)

2. The environment variable for standalone mode was renamed from BRAINTRUST_STANDALONE to BRAINTRUST_LANGSMITH_STANDALONE, but the examples in py/examples/langsmith/ still reference the old name. Users following eval_example.py or tracing_example.py will set BRAINTRUST_STANDALONE=1, which the new integration ignores.

braintrust-sdk-python/py/src/braintrust/integrations/langsmith/tracing.py

Lines 21 to 25 in 8c9d4a6

	def _langsmith_standalone_from_env() -> bool:
	value = os.environ.get("BRAINTRUST_LANGSMITH_STANDALONE")
	if value is None:
	return False
	return value.strip().lower() in {"1", "true", "yes", "on"}

braintrust-sdk-python/py/examples/langsmith/eval_example.py

Lines 22 to 24 in 8c9d4a6

	# Set BRAINTRUST_STANDALONE=1 to completely replace LangSmith with Braintrust
	standalone = os.environ.get("BRAINTRUST_STANDALONE", "").lower() in ("1", "true", "yes")

braintrust-sdk-python/py/examples/langsmith/tracing_example.py

Lines 21 to 23 in 8c9d4a6

	# Set BRAINTRUST_STANDALONE=1 to completely replace LangSmith with Braintrust
	standalone = os.environ.get("BRAINTRUST_STANDALONE", "").lower() in ("1", "true", "yes")