Add Admin API and sys procedure to describe bucket metadata

### Search before asking

- [x] I searched in the [issues](https://github.com/apache/fluss/issues) and found nothing similar.

### Motivation

Currently, bucket metadata can be inspected indirectly from internal metadata or client-side metadata, but there is no formal Admin API or SQL procedure for users and operators to describe bucket distribution.

This makes it hard to inspect bucket-level metadata such as leader, leader epoch, replicas, ISR, and partition-specific bucket assignment through a stable public interface.

### Solution

Add a formal Admin API and SQL procedure for describing bucket metadata.

The proposed Admin APIs are:

```java
CompletableFuture> describeBuckets(TablePath tablePath);

CompletableFuture> describeBuckets(
TablePath tablePath,
PartitionSpec partitionSpec);
```

The returned BucketInfo should include:

- table path
- table id
- partition id/name, if applicable
- bucket id
- leader id
- leader epoch
- replicas
- ISR

Expose the same functionality through Flink SQL procedure:

```sql
CALL sys.describe_buckets('db.table');

CALL sys.describe_buckets('db.table', 'partition_key=partition_value');
```

For non-partitioned tables, the procedure returns one row per bucket. For partitioned tables, the overload with `PartitionSpec` returns bucket metadata for the specified partition.

### Scope

This issue focuses on:

- Admin API
- `BucketInfo` DTO
- server RPC implementation
- partition-specific bucket description
- `CALL sys.describe_buckets`
- Admin IT and Flink procedure IT

CLI integration is intentionally left out of scope and can be handled in a follow-up issue/PR.

### Relation to existing work

This is related to #3360, but focuses on a different layer.

#3360 adds CLI table/database commands and displays bucket distribution from client metadata. This issue proposes a formal Admin/RPC API and SQL procedure for describing bucket metadata, including partition-scoped queries.

CLI support can be added later, preferably on top of the CLI framework introduced by #3360 if it is merged.

### Anything else?

_No response_

### Willingness to contribute

- [x] I'm willing to submit a PR!

Start by locating existing Admin API, server RPC, and Flink sys procedure patterns, then trace how bucket and partition metadata are represented. Review the proposed BucketInfo fields and the Admin and procedure integration-test areas. Done means both API overloads and CALL forms return the specified bucket metadata, including partition-scoped results, with Admin and Flink procedure IT coverage.

Written by the indexing model from the issue text.