MINOR: Improve endpoint information epoch management in Streams group coordinator

[lucasbru]

Fix endpoint information epoch management in Streams group coordinator

This commit addresses several issues in the endpoint information epoch
handling for Streams groups:

1. Conditional epoch bumping: Only increment endpointInformationEpoch
   when a member actually has a user endpoint defined, rather than on
   every member update. This prevents unnecessary epoch bumps for
   members without endpoints.

2. Initial epoch value: Change endpointInformationEpoch initialization
   from -1 to 0 to be consistent with standard epoch semantics.

3. Response epoch handling: Only include endpointInformationEpoch in
   responses when the group has been persisted, to better deal with the
   case where a group is newly created but the epoch information is
   not persisted (because it's soft state which is thrown away after
   the execution of the first heartbeat).

4. Endpoint-to-partitions building: Refactor
   maybeBuildEndpointToPartitions
   to consistently include both existing and new members' endpoint
   information. The previous implementation did not incldue the endpoint
   for a new member. Also return an empty list instead of null if the
   set of endpoints actually becomes empty.

The changes ensure that endpoint information is correctly propagated to
all group members during rebalancing, including when new members join
with user endpoints. Tests updated to reflect the new behavior and a
comprehensive test added to verify endpoint information includes both
existing and new members.

Reviewers: Bill Bejeckbbejeck@apache.org

[Copilot]

Pull Request Overview

This PR refactors endpoint information management in Streams group heartbeat responses by changing the initial endpoint information epoch from -1 to 0 and improving when endpoint information is included in responses. The key changes aim to ensure that endpoint information is properly included when members join and that the epoch handling is more intuitive.

Key Changes:

• Changed the default endpoint information epoch from -1 to 0 in StreamsGroup
• Modified logic to always return endpoint information for newly created groups (epoch 0)
• Simplified maybeBuildEndpointToPartitions to always return a list and properly include the updated member
• Added a new test case to verify endpoint information includes all members when a new member joins

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
GroupMetadataManagerTest.java	Updated test expectations to remove explicit endpoint epoch settings and added new test for multi-member endpoint scenarios
EndpointToPartitionsManager.java	Added new helper method maybeEndpointToPartitions to create endpoint mappings only when members have endpoints
StreamsGroup.java	Changed default endpointInformationEpoch from -1 to 0
GroupMetadataManager.java	Modified endpoint epoch response logic and refactored endpoint list building to properly include updated members

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[bbejeck]

Thanks for PR @lucasbru LGTM