[server] Add Cluster Health API implementation

[swuferhong]

Purpose

Linked issue: close #3399

• Add GetClusterHealth RPC to Coordinator that computes cluster health from in-memory state
• Track inactive leaders in CoordinatorContext (marked inactive on NotifyLeaderAndIsr send,
  marked active on successful response when responding server is still the leader)
• Handle send failures in CoordinatorRequestBatch by synthesizing error responses to clear
  pending inactive state
• Add client API Admin.getClusterHealth() with ClusterHealth / ClusterHealthStatus types
• Add ClusterHealthReadinessCheck CLI tool in fluss-dist (exit 0=GREEN, 1=not ready, 2=API unsupported)
• Add readiness-check.sh two-step readiness probe script (TCP + Cluster Health API)
  with first-boot detection and grace period for API-unsupported (mixed-version rolling upgrade)
• Wire tablet-server readiness probe to readiness-check.sh in Helm chart
• Add documentation for Helm deployment and upgrade guide

Brief change log

Tests

API and Format

Documentation