fix consumer group rewrite

[bubunyo]

No description provided.

[Copilot]

Pull request overview

This PR fixes consumer group topic rewriting by decoding and rewriting Kafka consumer group member blobs so that group subscriptions/assignments are rewritten consistently across JoinGroup, SyncGroup, and DescribeGroups flows.

Changes:

• Decode/encode consumer JoinGroup protocol metadata to prefix subscribed topics on requests and strip them for the leader on responses.
• Decode/encode consumer SyncGroup member assignments to prefix topics on requests and strip them in responses back to members.
• Extend DescribeGroupsResponseOut to also strip prefixed topic names inside member subscription / assignment blobs; add focused unit tests for these cases.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
rewrite/groups.go	Adds consumer-group blob decoding helpers and applies topic rewriting to JoinGroup/SyncGroup and DescribeGroups member blobs.
rewrite/groups_test.go	Adds helpers + new tests validating topic rewriting/stripping behavior for member metadata/assignments and non-consumer passthrough.
proxy/rewrite_groups.go	Ensures JoinGroup/SyncGroup responses are post-processed so clients see client-space topics in leader/member blobs.

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