fix: support groups key for capture events

[marandaneto]

💡 Motivation and Context

Fixes #71.

capture() documents and examples have allowed passing group context with the groups key, but PostHog ingestion associates events with groups via the $groups event property. This PR preserves backwards compatibility by promoting an explicitly supplied groups key to $groups before enqueueing the capture event, while keeping existing $groups behavior unchanged.

💚 How did you test it?

• ./vendor/bin/phpunit --no-coverage test/PostHogTest.php --filter '/testCaptureCopiesGroupsKeyToDollarGroupsProperty|testCaptureKeepsDollarGroupsCompatibility/'
• ./vendor/bin/phpunit --no-coverage
• composer api:check
• ./vendor/bin/phpcs --standard=phpcs.xml --warning-severity=0 lib/Client.php test/PostHogTest.php
• git diff --check

📝 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: Human-driven (agent-assisted)

This PR was implemented with the pi coding agent. We investigated the PostHog ingestion path and confirmed that event grouping is read from properties.$groups; top-level groups is not consumed by backend event ingestion. The chosen fix keeps the existing public groups capture input working by promoting it to $groups, without changing feature flag groups behavior or removing existing $groups compatibility.

[greptile-apps]

Prompt To Fix All With AI

Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 1
test/PostHogTest.php:460-502
**Prefer a parameterised test over two separate methods**

The two new methods (`testCaptureCopiesGroupsKeyToDollarGroupsProperty` and `testCaptureKeepsDollarGroupsCompatibility`) cover the same logical scenario — capture with a groups key ends up in `properties['$groups']` — and differ only in which input key is used (`groups` vs `$groups`). Per the project's stated preference for parameterised tests, these should be collapsed into a single `@dataProvider`-driven method. Separate methods make it harder to add coverage for additional variants (e.g. both keys supplied simultaneously) without more copy-paste.

_{Reviews (1): Last reviewed commit: "fix: support groups key for capture even..." | Re-trigger Greptile}