fix: Clear feature flag called cache on shutdown

[marandaneto]

💡 Motivation and Context

Feature flag call deduplication state currently remains in memory after shutdown, which can leak stale called-cache state across a client shutdown/reuse lifecycle. This clears the cache during shutdown while holding the existing cache mutex.

💚 How did you test it?

• bundle exec rspec spec/posthog/client_spec.rb
• git diff --check

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed. (Not needed.)
• No breaking change or entry added to the changelog.

If releasing new changes

• Added a changeset file (.changeset/quiet-cycles-shutdown.md)

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

An agent prepared this PR from the dedicated fix/ff-called-cache-shutdown worktree at the user's direction. The change adds a guarded shutdown-time clear of the feature flag call dedupe cache, a regression spec for that behavior, and a patch changeset for posthog-ruby.

Validation included the client spec file and whitespace checks. A local pnpm changeset status --since=main validation attempt failed because dependencies are not installed in this worktree (changeset executable missing), but the changeset file was added using the repository's Changesets format.

[greptile-apps]

_{Reviews (1): Last reviewed commit: "fix: Clear feature flag called cache on ..." | Re-trigger Greptile}

[greptile-apps]

_{Reviews (2): Last reviewed commit: "chore: release posthog-rails for shutdow..." | Re-trigger Greptile}