[Bug] LangSmith tracing fails for non-US users + env changes not picked up without re-sourcing

[gloriamacia]

Description

Two related setup issues that prevent LangSmith tracing from working,
particularly for users outside the US.

Issue 1: Hardcoded US endpoint

variables.env hardcodes LANGSMITH_ENDPOINT to the GCP US endpoint.
Users who registered on EU, APAC, or AWS deployments get a persistent
403 Forbidden on all trace ingestion calls with no clear error message
pointing to the cause.

LangSmith has four regional API endpoints:

Region	Endpoint
GCP US (default)	https://api.smith.langchain.com
GCP EU	https://eu.api.smith.langchain.com
GCP APAC	https://apac.api.smith.langchain.com
AWS US	https://aws.api.smith.langchain.com

Additionally, older versions of the langsmith client check LANGCHAIN_ENDPOINT
instead of LANGSMITH_ENDPOINT, so both should be set.

Issue 2: env changes not picked up without re-sourcing

The workshop instructions do not mention that after editing variables.env,
users must run:

source /project/variables.env

before running langgraph dev. Without this step, the old environment
variables remain active in the terminal session and changes have no effect.

Fix

• Add a regional comment block to variables.env listing all 4 endpoints
• Mirror LANGSMITH_ENDPOINT to LANGCHAIN_ENDPOINT for client compatibility
• Add source /project/variables.env as an explicit step in setup instructions

A PR with this fix is forthcoming.