databendlabs / openraft / 19489036785

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change: in 0.10.0: `Raft::as_leader()` returns `Result` with `ForwardToLeader` error

Changed the return type from `Option<Leader<C>>` to `Result<Leader<C>, ForwardToLeader<C>>`
to provide useful information when the node is not a leader. The error now contains
the current known leader's node ID and node info for request forwarding.

Changes:
- Change `Raft::as_leader()` to return `Result<Leader<C>, ForwardToLeader<C>>`
- Return `ForwardToLeader` with leader info when node is not leader
- Return `ForwardToLeader::empty()` when no leader is known

Upgrade tip:

Replace pattern matching on `Option` with `Result`:
- `if let Some(leader) = raft.as_leader()` → `if let Ok(leader) = raft.as_leader()`
- `raft.as_leader().is_none()` → `raft.as_leader().is_err()`
- `raft.as_leader().expect("msg")` → `raft.as_leader().expect("msg")` (unchanged)

To get leader info for forwarding:
```rust
match raft.as_leader() {
    Ok(leader) => { /* handle as leader */ }
    Err(forward) => {
        // forward.leader_id: Option<NodeId>
        // forward.leader_node: Option<Node>
    }
}
```

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