fix(flags): store setOnce properties in locally persisted feature flag props

[havenbarnes]

Changes

Context: https://posthoghelp.zendesk.com/agent/tickets/24489

Include setOnce properties in the locally persisted feature flag person properties: https://posthog.com/docs/feature-flags/adding-feature-flag-code#automatic-overrides

These are stored in local storage, but only used by feature flags for passing into the /decide call. This means that we should be safe from setOnce properties inadvertently overwriting previous property values.

Checklist

• Tests for new code (see advice on the tests we use)
• Accounted for the impact of any changes across different browsers
• Accounted for backwards compatibility of any changes (no breaking changes in posthog-js!)

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Name	Status	Preview	Updated (UTC)
posthog-js	✅ Ready (Inspect)	Visit Preview	Feb 4, 2025 11:28pm

[greptile-apps]

PR Summary

This PR modifies the feature flag property persistence in posthog-js to include setOnce properties, ensuring they are stored locally and only used for /decide calls without affecting existing property values.

• Modified identify and setPersonProperties methods in /src/posthog-core.ts to merge userPropertiesToSetOnce with userPropertiesToSet
• Properties are only used for feature flag /decide calls, preventing unintended overwrites of existing values
• Changes maintain backwards compatibility and work consistently across browsers
• Functionality verified through existing feature flag and identify tests in /functional_tests/ and /playwright/

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[github-actions]

Size Change: +312 B (+0.01%)

Total Size: 3.3 MB

Filename	Size	Change
dist/array.full.es5.js	268 kB	+30 B (+0.01%)
dist/array.full.js	371 kB	+30 B (+0.01%)
dist/array.full.no-external.js	369 kB	+30 B (+0.01%)
dist/array.js	183 kB	+34 B (+0.02%)
dist/array.no-external.js	181 kB	+30 B (+0.02%)
dist/main.js	184 kB	+34 B (+0.02%)
dist/module.full.js	371 kB	+30 B (+0.01%)
dist/module.full.no-external.js	369 kB	+30 B (+0.01%)
dist/module.js	183 kB	+34 B (+0.02%)
dist/module.no-external.js	182 kB	+30 B (+0.02%)

ℹ️ View Unchanged

Filename	Size
dist/all-external-dependencies.js	215 kB
dist/customizations.full.js	13.8 kB
dist/dead-clicks-autocapture.js	14.5 kB
dist/exception-autocapture.js	9.48 kB
dist/external-scripts-loader.js	2.64 kB
dist/recorder-v2.js	115 kB
dist/recorder.js	115 kB
dist/surveys-preview.js	69.4 kB
dist/surveys.js	72.4 kB
dist/tracing-headers.js	1.76 kB
dist/web-vitals.js	10.4 kB

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