fix: Unsubscribe the provider's listeners on shutdown

[kinyoklion]

The provider's data source status and flag change listeners stayed registered on the LaunchDarkly client after shutdown, so a provider which the OpenFeature SDK had already replaced could still emit events. They are now removed before the client is closed.

Event deduplication and the events emitted after a failed initialization, which this PR previously carried, now live in #36 instead; this branch has been merged with that one and keeps only the shutdown change. Review #36 first.

Found during the weekly OpenFeature provider audit.

Implementation details

Requirements

• I have added test coverage for new or changed functionality
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions

Related issues

None.

Describe the solution you've provided

The listeners the constructor registers are kept on the provider so that shutdown can pass them to remove_listener on the data source status provider and the flag tracker before closing the client.

Describe alternatives you've considered

Relying on close alone: the LaunchDarkly client stops notifying listeners once it is closed, but the provider is the owner of those subscriptions and a closed client is not a guarantee the OpenFeature SDK makes about a replaced provider.

Additional context

Testing: bundle exec rspec (83 examples) and bundle exec rubocop on Ruby 3.4.5. The shutdown spec now asserts both listeners are removed as well as the client being closed.

Link to Devin session: https://app.devin.ai/sessions/fe1eb757fe694ef79f3d09f6307d4b47
Open in Devin Desktop: https://app.devin.ai/desktop/session/fe1eb757fe694ef79f3d09f6307d4b47?variant=devin
Requested by: @kinyoklion

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment, CI, and merge conflict monitoring

[devin-ai-integration]

@cursor review

[cursor]

Cursor Bugbot has reviewed your changes using default effort and found 1 potential issue.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, have a team admin enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit e2f07de. Configure here.}