fix: return booleans from immediate batch flushes

[marandaneto]

💡 Motivation and Context

Fixes #57.

When batch_size is 1, queued calls flush immediately. LibCurl::flushBatch() returned the raw batch endpoint response body instead of a boolean, so PostHog::alias() could return a JSON string for immediate flushes but true when the event was only queued. The socket consumer also had related non-boolean/failure return paths during immediate flushes.

💚 How did you test it?

• ./vendor/bin/phpunit --no-coverage --colors=never

📝 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

• Added the release label to the PR
• Added exactly one version bump label: bump-patch, bump-minor, or bump-major