feat: add stream-oriented AppendEntries API for pipelined replication

[drmingdrmer]

Changelog

feat: add stream-oriented AppendEntries API for pipelined replication

Add Raft::stream_append() method that allows processing multiple AppendEntries
requests through a pipelined stream interface. This enables efficient batched
log replication with backpressure support via a bounded channel.

Example usage:

use std::pin::pin;
use futures::StreamExt;

let input = futures::stream::iter(vec![request1, request2, request3]);
let mut output = pin!(raft.stream_append(input));

while let Some(result) = output.next().await {
    match result {
        Ok(log_id) => println!("Flushed: {:?}", log_id),
        Err(err) => {
            println!("Error: {}", err);
            break;
        }
    }
}

Changes:

• Add Raft::stream_append() for stream-based AppendEntries processing
• Add StreamAppendError with Conflict and HigherVote variants
• Add StreamAppendResult type alias for stream item results
• Add AppendEntriesResponse::into_stream_result() conversion method
• Add ProtocolApi::stream_append() internal implementation
• Add integration tests for success, conflict, and higher vote scenarios

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This change is 

[xp-trumpet]

@xp-trumpet reviewed 8 of 8 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @drmingdrmer)