[client] Expose all coordinator servers (leader and standbys) via the client API

[affo]

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• I searched in the issues and found nothing similar.

Motivation

Since coordinator HA (1.0), a cluster may run several coordinator servers, but the client API still surfaces exactly one: MetadataResponse carries a single coordinator_server (the current leader), and Admin#getServerNodes() returns that node plus the alive tablet servers. Standby coordinators are invisible to clients — they reject every request except apiVersions with NotCoordinatorLeaderException.

Operational tooling (consoles, operators, CLIs) needs to display and verify the HA topology: how many coordinators exist, which one is the leader, and whether the standbys are alive. Today the only sources are out-of-band — ZooKeeper or the deployment platform (for example, Kubernetes pods) — which not every client can or should reach.

Solution

1. Expose all registered coordinator servers with a role marker (leader/standby) through the client API — either by extending getServerNodes/MetadataResponse, or as part of the cluster-info API proposed in [rebalance] Change the implementation of Admin#getServerNodes() as introducing a new API to get cluster information #1389, or as a dedicated Admin#describeCoordinators().
2. Include a liveness signal per coordinator (for example, presence in the election group), so a dead standby is distinguishable from a live one.

Related: #4105 needs a readiness signal that is valid for standby coordinators; a client-visible role/liveness API could serve that consumer too. The request handler's blanket rejection of all non-leader requests is already marked as provisional in a code comment (FlussRequestHandler) — serving this (read-only) call from a standby would be consistent with narrowing that check.

Anything else?

Affected areas: fluss-rpc (FlussApi.proto: metadata or a new describe message), fluss-server (CoordinatorService, election-group state), fluss-client (Admin).

Willingness to contribute

• I'm willing to submit a PR!

[striver619]

@affo

i already add this pr: #4111 about coordinator ha