consumer: return rebalance event via Poll() when appRebalanceEnable && no RebalanceCb

[aryanrahar]

What

Return the rebalance event from handleRebalanceEvent when
go.application.rebalance.enable == true and no RebalanceCb is set,
regardless of whether an events channel is used.

Why: In this configuration, Poll()-based consumers did not receive the
rebalance event and could hang until a callback was wired. This change makes
Poll()-based apps receive AssignedPartitions / RevokedPartitions so they
can call Assign() / Unassign() to ack, aligning behavior with the config’s
intent. No API changes; callback path unchanged.

Checklist

• Contains customer facing changes? Including API/behavior changes
  (Behavioral bug fix only; no API change; backward-compatible.)
• Did you add sufficient unit test and/or integration test coverage for this PR?
  (No new unit test in this PR; integration test typically requires a broker.
  See “Test & Review” for manual steps. I can follow up with a tagged
  integration test if desired.)

References

Fixes #1457

Test & Review

Local sanity:

• go build ./... ✅
• go test ./... ✅ (unit tests; no broker required)

Manual verification (requires reachable Kafka):

1. Create a consumer with:
   "bootstrap.servers": "",
   "group.id": "rebalance-poll-it",
   "auto.offset.reset": "earliest",
   "go.application.rebalance.enable": true
   (No RebalanceCb; use Poll() — no events channel.)

2. Subscribe() to a topic and call Poll() in a loop.

3. Before this change: no rebalance event arrives via Poll().
   After this change: an AssignedPartitions/RevokedPartitions event is
   returned by Poll().

4. Call Assign() / Unassign() accordingly; consumer proceeds and closes cleanly.

Open questions / Follow-ups

• I can send a tiny follow-up PR to update the comment above
  handleRebalanceEvent to explicitly document this behavior (refs Incorrect documentation on handleRebalanceEvent #1421).
• If maintainers prefer, I can also add a small integration test behind a build
  tag (requires kafka/testconf.json and a broker).

[aryanrahar]

Can you review or check my changes!