A: Code quality improvements - fix clippy warnings

- Remove unused imports in server.rs (Body, HeaderValue, RwLock)
- Remove unused imports in forward_acl.rs (tests still need Ipv4Addr)
- Remove unused imports in host_key.rs (Read, Write)
- Remove unused imports in kex_exchange.rs (HostKeyType)
- Remove unused imports in known_hosts.rs (tests need Ipv4Addr)
- Remove unused imports in multiplex.rs (Arc)
- Auto-fix other unused imports via clippy --fix

Tests: 303 passed, 0 failed (4 new tests added)

markbase-core/src/s3_auth.rs

@@ -185,7 +185,7 @@ fn uri_encode(input: &str, encode_slash: bool) -> String {
         .collect()
 }
 
-fn build_canonical_query_string(headers: &HeaderMap) -> String {
+fn build_canonical_query_string(_headers: &HeaderMap) -> String {
     // For S3, query string is typically empty for basic operations
     // This can be extended for presigned URLs
     String::new()