fix: include group context in $feature_flag_called dedupe key

[gustavohstrassburger]

💡 Motivation and Context

In Client::captureFlagCalledIfNeeded (lib/Client.php), the per-distinct_id dedupe element only included the distinct ID under the flag key. For group-scoped flags this meant that when the same user was evaluated under a different group, no new $feature_flag_called event was fired — causing per-group exposure undercount for experiments scoped to a group key.

Mirrors the posthog-node fix in PostHog/posthog-js#3658 (which closes PostHog/posthog-js#3651). Both SDKs share the same dedupe shape, so backend evaluation needs the same change.

💚 How did you test it?

Added three PHPUnit tests in test/FeatureFlagEvaluationsTest.php:

• testFeatureFlagCalledFiresPerGroupContext — same user, same flag, two different group contexts → two events fired with the expected $groups payloads.
• testFeatureFlagCalledDedupesAcrossRepeatedCallsUnderSameGroup — repeated access under the same group → one event.
• testFeatureFlagCalledDedupesAcrossGroupKeyOrder — same groups passed with different array insertion order → still one event (canonicalized via ksort + json_encode).

vendor/bin/phpunit is green (311 tests pass).

📝 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

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Created with PostHog Code

[greptile-apps]

Prompt To Fix All With AI

Fix the following 2 code review issues. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 2
lib/Client.php:984-985
`json_encode` returns `false` on encoding failure (e.g., non-UTF-8 bytes in a group value). PHP silently casts `false` to `''`, so every failing encoding produces the suffix `'_'` — all such groups collapse into the same dedup bucket and subsequent `$feature_flag_called` events are silently dropped. Adding a `JSON_THROW_ON_ERROR` flag makes the failure explicit.

```suggestion
        ksort($groups);
        return '_' . json_encode($groups, JSON_UNESCAPED_SLASHES | JSON_UNESCAPED_UNICODE | JSON_THROW_ON_ERROR);
```

### Issue 2 of 2
test/FeatureFlagEvaluationsTest.php:609-655
**Prefer parameterised tests for the two dedup-count cases**

`testFeatureFlagCalledDedupesAcrossRepeatedCallsUnderSameGroup` and `testFeatureFlagCalledDedupesAcrossGroupKeyOrder` both set up a different call pattern, flush, count matching events, and assert exactly 1. Per the team's preference, these share enough structure to be combined under a single `@dataProvider` — each data set would supply the group arrays to pass on each call and the expected event count, keeping the assertion logic in one place.

_{Reviews (1): Last reviewed commit: "chore: add changeset" | Re-trigger Greptile}