Add Tests

[motero2k]

Now that we've migrated to TypeScript and some functionality is relatively stable (even though we're still in v0), it would be valuable to introduce integration tests that validate the real-world usage of the package.

What to do

Create integration tests that:

1. Spin up a sample test app with our package installed.
2. Ensure that the /telemetry endpoint is reachable.
3. Perform a POST request to /telemetry.
4. Verify that the list of collected spans has been updated accordingly.

These tests should run against the built output, both for:

• ESM build
• CJS build

This ensures that new features and the transpilation don't break any runtime behavior.

Why this matters

• We're aiming for a stable v0 experience that teams can adopt with confidence.
• Validating actual runtime behavior is key to catching integration-level bugs.
• Ensures compatibility across Node.js module systems.