feat(surveys): add support for schedule 'always' setting

[adboio]

Add support for the 'schedule: always' survey setting which enables surveys to display every time conditions are met, regardless of prior exposure.

Changes:

• Add schedule property to PostHogSurvey model
• Add PostHogSurveySchedule enum with once/recurring/always/unknown cases
• Update canActivateRepeatedly to return true when schedule is 'always'
• Add tests for schedule-based repeated activation

Closes #446

💡 Motivation and Context

💚 How did you test it?

📝 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.

[github-actions]

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Please add an entry to CHANGELOG.md to the "Next" section. Make sure the entry includes this PR's number.
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## Next
- add support for schedule 'always' setting ([#454](https://github.com/PostHog/posthog-ios/pull/454))

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