Add realistic e2e test app for LangGraph #24756

Add an e2e test app that uses the LangGraph integration the way a real user does, and that sends the data to a real Sentry project.

Current state: No e2e test exists for this integration.

Requirements:

• Send real requests to the model provider (OpenRouter is OK).
• Use a real E2E_TEST_DSN with no tunnel, so the data goes to Sentry.
• Read the gen_ai spans back through the Sentry API and check them, the same way the *-send-to-sentry apps do.
• Add a variant that runs against the latest version of the library.