[client] Add Admin API to describe buckets

[fhan688]

Purpose

Linked issue: #3436

Fluss currently does not provide a public Admin API for retrieving bucket-level metadata, including replica placement and leader/ISR state. This information is required by administrative tools and by the planned Flink sys.describe_buckets procedure.

This is the first of two PRs planned for #3436. It introduces the core Admin API, RPC contract, and server-side implementation. A follow-up PR will add the Flink procedure and close the issue.

Brief change log

• Add the public BucketInfo result type, exposing the table path and ID, optional partition ID and name, bucket ID, optional leader and leader epoch, replicas, and ISR.
• Add Admin#describeBuckets(TablePath) and Admin#describeBuckets(TablePath, PartitionSpec).
• Add the dedicated public DESCRIBE_BUCKETS RPC (API key 1065), including request/response messages, gateway wiring, and client-side conversion.
• Support non-partitioned tables, all partitions of a partitioned table, and complete or partial partition-spec filtering.
• Enforce the table-level DESCRIBE permission and preserve table/partition validation exceptions at the Admin API boundary.
• Batch ZooKeeper reads for partition registrations and bucket metadata, exclude the historical partition, and return results deterministically by partition name and bucket ID.
• Add unit, integration, authorization, RPC conversion, and ZooKeeper coverage.

The Flink sys.describe_buckets procedure is intentionally excluded from this PR and will be added in the follow-up PR.

Tests

• BucketInfoTest
• ClientRpcMessageUtilsTest
• ApiKeysTest
• ApiManagerTest
• ZooKeeperClientTest#testPartition
• DescribeBucketsITCase
  • non-partitioned table bucket metadata
  • partitioned table queries, complete/partial partition filters, validation errors, deterministic ordering, and ISR metadata
• FlussAuthorizationITCase#testDescribeTableOperation
• fluss-flink-common test compilation for Admin interface compatibility
• Spotless, RAT, Checkstyle, and git diff --check

API and Format

• Public Java API: Yes. This PR adds the @PublicEvolving BucketInfo class and two new methods to Admin.
• RPC protocol: Yes. This PR adds the public DESCRIBE_BUCKETS API (key 1065) and its protobuf messages.
• Storage format: No.
• The existing PbBucketMetadata wire message is unchanged, avoiding overlap with the separate ISR metadata work in feat: shell tool #3360.

Documentation

Public API Javadocs are included for BucketInfo and the new Admin methods.

No user-facing Flink procedure documentation is added in this PR because the procedure is outside this PR's scope. The follow-up PR will add the procedure, integration tests, and corresponding documentation.