fix(flags): pass the resolved polling interval to the definitions poll timer

[flivni]

💡 Motivation and Context

feature_flags_polling_interval is documented as "Defaults to 30" and the poller computes @polling_interval = polling_interval || 30, but the value was not actually used, and the 60-second concurrent-ruby was used instead.

This PR fixes the above.

Behavior change worth reviewing: Halves the current effective default. Alternatively, we could update the documented default to 60s.

💚 How did you test it?

• New "definitions polling interval" specs (2 examples) assert the TimerTask's effective execution_interval: 30 when the option is unset, and the configured value when provided. Before the fix, the default case reports 60.
• Full suite: 616 examples, 0 failures. rubocop clean. rake public_api:check passes with the snapshot regenerated for the one new constant.

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran pnpm changeset to generate a changeset file

🤖 Agent context

Autonomy: Agent-written, human-directed and reviewed

This bug was found by Claude Code (Fable 5) in a code review on a different PR. All code was written by Claude. My contribution is limited to the "Motivation and Context" paragraph and this paragraph here. I've read the code and I fully understand it.

[turnipdabeets]

LGTM but @flivni both PRs need a verified signature to merge. Mind adding those? We'll also need to rebase #221 after this one merges

[flivni]

@turnipdabeets Done — signatures added on both PRs. Can rebase #221 once this one merges.