Add realistic e2e test app for Flue #24753

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

Current state: node-flue and cloudflare-flue send real requests through OpenRouter. But the SDK sends the spans to the local event proxy (tunnel: localhost:3031), so nothing gets to Sentry.

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.