[Test]: Automated test for package installation and imports resolution

Develop an automated test for #287, see good starting point shared by the author: https://github.com/a2aproject/a2a-js/issues/287#issuecomment-3732255191.

The test should do `npm pack` and `npm install` in various setups. Fake usage code should import symbols from all entry points.

It should be possible to extend the check added in #210.

Start by reviewing the existing check from issue #210 and the detailed comment in issue #287. The test needs to simulate packaging and installing the SDK via `npm pack` and `npm install`, then verify imports from all entry points work. Look for existing test files related to packaging or installation to understand the project's test structure. The goal is to create a test that can be run as part of the CI pipeline.

Written by the indexing model from the issue text.