`binding-kafka`: extend `KafkaMetaDataEx` with replica and ISR partition detail

[jfallows]

Summary

Extend KafkaMetaDataEx and its KafkaPartition element to carry full
partition metadata — replicas, isr, and topic-level replicationFactor —
as returned by the Kafka Metadata API (API key 3). Required by the
mcp_kafka tools list_topics, describe_topic, and cluster_overview.

Current types (kafka.idl)

struct KafkaPartition
{
    int32 partitionId;
    int32 leaderId;
}

struct KafkaMetaDataEx
{
    KafkaPartition[] partitions;
}

Required changes

Add a new KafkaPartitionMetadata struct carrying full partition detail, and
extend KafkaMetaDataEx to include topic-level replication factor:

struct KafkaPartitionMetadata
{
    int32 partitionId;
    int32 leaderId;
    int32[] replicas;       // all replica broker IDs
    int32[] isr;            // in-sync replica broker IDs
}

struct KafkaMetaDataEx     // replaces existing KafkaMetaDataEx
{
    int16 replicationFactor;
    KafkaPartitionMetadata[] partitions;
}

KafkaPartition remains unchanged — it is used elsewhere (e.g.
KafkaTopicPartition, KafkaPartition in consumer assignment structs).
KafkaMetaDataEx is updated in place; existing callers that only read
partitionId and leaderId must be updated to use KafkaPartitionMetadata.

Kafka API

Metadata (API key 3) — already in KafkaApi enum and KafkaBeginEx.

Spec test coverage

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

• Metadata response with replicationFactor, replicas, and isr correctly
  populated in KafkaMetaDataEx
• isr subset of replicas for an under-replicated partition

Additional context

• kafka.idl
• Kafka Protocol — Metadata