fix: remove ignored batch event fields

[marandaneto]

💡 Motivation and Context

The backend /batch/ endpoint ignores several legacy top-level fields on individual events (type, library, library_version, library_consumer, and send_feature_flags). Keeping them in SDK payloads adds noise and can confuse which fields are canonical.

This change strips those deprecated top-level fields before sending events and keeps the backend-supported canonical SDK metadata in event properties instead. For backwards compatibility, legacy top-level library, library_version, and library_consumer values are still used as event-property fallbacks when the canonical $lib, $lib_version, or $lib_consumer values are absent. Top-level type is ignored and not used as a fallback. The HTTP User-Agent now always uses the official PostHog::LIBRARY / PostHog::VERSION constants regardless of event metadata.

💚 How did you test it?

• composer run api:check
• php -l on changed PHP files
• ./vendor/bin/phpunit --testsuite posthog-php (tests pass: 339 tests / 3391 assertions; command exits non-zero because PHPUnit reports no code coverage driver)
• 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)

An AI coding agent made the implementation and validation changes at the direction of the maintainer. The backend batch schema was checked first, then the SDK was updated to stop sending ignored per-event top-level fields while preserving canonical $lib, $lib_version, and $lib_consumer properties. Backwards compatibility was preserved for legacy top-level SDK metadata values as event-property fallbacks when canonical values are absent; type remains a no-op. Existing payload assertions and file-consumer expectations were updated to match the canonical shape.

[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
lib/Client.php:349-353
**`groups: []` still leaks out of `identify()` and `alias()` payloads**

The `message()` helper (line 1690-1692) always injects `"groups" => []` as a default, and `capture()` explicitly unsets it at line 291. But `identify()` and `alias()` have no matching `unset($message["groups"])`, so every identify/alias event still ships `"groups":[]` to the batch endpoint — the very noise this PR aims to eliminate. The same one-liner added to `capture()` would fix both methods.

_{Reviews (1): Last reviewed commit: "fix: remove ignored batch event fields" | Re-trigger Greptile}

[greptile-apps]

_{Reviews (2): Last reviewed commit: "fix: remove ignored batch event fields" | Re-trigger Greptile}