fix: include group context in $feature_flag_called dedupe key

[gustavohstrassburger]

💡 Motivation and Context

In _capture_feature_flag_called_if_needed (lib/posthog/client.rb), the per-distinct_id dedupe key only included the flag key and response. 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 rspec examples covering the new behavior in spec/posthog/client_spec.rb:

• $feature_flag_called fires per group context (group-scoped dedup) — same user, same flag, two different group contexts → two events.
• $feature_flag_called dedupes across repeated calls under the same group context — repeated access under the same group → one event.
• $feature_flag_called dedupes when same groups are passed in different key order — same groups passed with keys in a different order → one event (canonicalized via groups.sort.to_json).

All 78 existing client tests pass, and all 35 feature_flag_evaluations_spec 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 1 code review issue. Work through them one at a time, proposing concise fixes.

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### Issue 1 of 1
spec/posthog/client_spec.rb:619-682
**Duplicated test setup across all three new examples**

The `api_feature_flag_res` hash, `stub_request`, and `Client.new` lines are copy-pasted verbatim in each of the three new `it` blocks. Extracting them into `let`/`before` inside a shared `context` would satisfy the OnceAndOnlyOnce rule and align with the team preference for parameterised tests. The deduplication pair (same-context → one event, different-context → two events) are also natural candidates for a parameterised example to remove repetition between tests 2 and 3.

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

[ioannisj]

One bot comment but not a blocker I believe