ci: validate v2 PyPI release installs

[tauhid621]

Validate v2 releases with pip installs on Linux, macOS, and Windows, checking dependencies, CLI version, and the Python API. Gate release announcements on validation success.

Validation: workflow syntax and mocked install/version checks passed. Live installs will run during a release.

[cla-bot]

Thank you for your pull request and welcome to our community. We could not parse the GitHub identity of the following contributors: test-bot.
This is most likely caused by a git client misconfiguration; please make sure to:

1. check if your git client is configured with an email to sign commits git config --list | grep email
2. If not, set it up using git config --global user.email email@example.com
3. Make sure that the git commit email is configured in your GitHub account settings, see https://github.com/settings/emails