`binding-kafka`: add `DescribeGroups` (API key 15) support

[jfallows]

Summary

Add DESCRIBE_GROUPS (API key 15) to the KafkaApi enum and
KafkaRequestBeginEx / KafkaResponseBeginEx unions. Required by the
mcp_kafka describe_consumer_group tool.

Required changes to kafka.idl

Add to KafkaApi enum:

DESCRIBE_GROUPS (15)

Add new request and response structs:

struct KafkaDescribeGroupMember
{
    string16 memberId;
    string16 clientId;
    string16 clientHost;
    varint32 metadataLen;
    octets[metadataLen] metadata = null;
    varint32 assignmentLen;
    octets[assignmentLen] assignment = null;
}

struct KafkaDescribeGroupInfo
{
    int16 error;
    string16 groupId;
    string16 groupState;
    string16 protocolType;
    string16 protocol;
    KafkaDescribeGroupMember[] members;
}

struct KafkaDescribeGroupsRequestBeginEx
{
    string16[] groupIds;
    uint8 includeAuthorizedOperations;
}

struct KafkaDescribeGroupsResponseBeginEx
{
    int32 throttle;
    KafkaDescribeGroupInfo[] groups;
}

Add to KafkaRequestBeginEx union:

case 15: kafka::stream::KafkaDescribeGroupsRequestBeginEx describeGroups;

Add to KafkaResponseBeginEx union:

case 15: kafka::stream::KafkaDescribeGroupsResponseBeginEx describeGroups;

Notes

Per-partition lag calculation for describe_consumer_group is the
responsibility of mcp_kafka, not binding-kafka. binding-kafka provides
raw group membership and assignment data; mcp_kafka combines this with
KafkaOffsetFetchBeginEx and latest offsets from KafkaFetchBeginEx to
compute lag.

Spec test coverage

Add scenarios to specs/binding-kafka.spec/ covering:

• DescribeGroups for a single active group with members and assignments
• DescribeGroups for an empty/dead group
• DescribeGroups for multiple groups in a single request
• Error response for unknown group

Additional context

• kafka.idl
• Kafka Protocol — DescribeGroups