fix: include distinct_id on $create_alias properties

[o-mid]

💡 Motivation and Context

$create_alias already sets distinct_id on the event envelope. Some pipelines only read properties, so they miss the pre-alias identity. This mirrors what other PostHog SDKs emit and covers aliasStateless as well.

Note: #819 proposes the same change for the main alias() path; this branch also updates the stateless API and adds a regression test there.

💚 How did you test it?

• :posthog:test — PostHogTest.captures an alias event, PostHogStatelessTest.aliasStateless creates alias event (local Gradle blocked on missing AGP 8.9.1 in this environment; CI expected to run full suite).

📝 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

Made with Cursor

[o-mid]

Closing in favor of #819, which already covers the PostHog.alias() path and the same PostHogTest assertion against the sdk-spec alias contract.

The only delta here was aliasStateless + PostHogStatelessTest — left on #819 as a follow-up comment so we do not maintain two changesets for the same fix.