feat(conductor): add demote-voter to conductor rpc

[KyrinCode]

Description

This PR exposes the DemoteVoter method via the conductor RPC API, allowing operators to demote a voting member to a non-voting member without removing and re-adding the node.

Problem

The consensus layer already has a DemoteVoter method implemented, but it was not exposed through the RPC API. Without this RPC endpoint, operators who wanted to demote a voter to nonvoter had to:

1. Remove the server via conductor_removeServer
2. Re-add it via conductor_addServerAsNonvoter
   This workaround has a significant drawback: when a node is removed, Raft's default behavior causes it to shut down, requiring a process restart before re-adding.

Solution

Expose the existing DemoteVoter consensus method through the RPC API as conductor_demoteVoter. This allows:

• Direct demotion of a voter to nonvoter in a single RPC call
• No Raft shutdown or process restart required
• Seamless role transitions within the cluster

Changes

• Added DemoteVoter method to rpc/api.go interface
• Added DemoteVoter implementation in rpc/backend.go
• Added DemoteVoter client method in rpc/client.go
• Added DemoteVoter handler in conductor/service.go

Usage

curl -X POST http://<conductor>:8547 \
  -d '{"jsonrpc":"2.0","method":"conductor_demoteVoter","params":["<server-id>", 0],"id":1}'

Tests

Manual testing was performed:

1. Started a 3-node conductor cluster with all nodes as voters
2. Called conductor_demoteVoter on one node
3. Verified the node transitioned from voter to nonvoter
4. Verified cluster membership reflected the change
5. Verified the demoted node continued to receive log replication
   This is a straightforward RPC exposure of an existing consensus method. The underlying DemoteVoter functionality is already tested as part of HashiCorp Raft.

Additional context

Note: If the leader is demoted, it will trigger a new leader election. This is expected Raft behavior.

Metadata

Complements existing cluster management APIs (addServerAsVoter, addServerAsNonvoter, removeServer)