[Store] Fix P2PClientService::Put silently swallowing write errors (#1825)

* fix: propagate PutViaRoute error in P2PClientService::Put

Put() logged errors from PutViaRoute but always returned success (empty
expected), causing callers to believe the write succeeded when the
underlying route write actually failed with SEGMENT_NOT_FOUND or
NO_AVAILABLE_HANDLE.

Return the error for non-idempotent failures while still treating
REPLICA_NUM_EXCEEDED / REPLICA_ALREADY_EXISTS as success (object already
stored).

Fixes #1817

* fix: also capture remote write errors in PutViaRoute result

Address review feedback from Gemini Code Assist:

1. PutViaRoute: assign write_result error to result before continue
   in the remote write path. Previously, if all remote candidates
   failed, result stayed default-initialized (success) because only
   local write failures were captured.

2. Put: simplify return to use result directly instead of
   re-wrapping with tl::unexpected.

Author: he-yufeng

mooncake-store/src/p2p_client_service.cpp

@@ -458,6 +458,7 @@ tl::expected<void, ErrorCode> P2PClientService::PutViaRoute(
                     write_result.error() != ErrorCode::REPLICA_ALREADY_EXISTS) {
                     LOG(WARNING) << "Remote write to " << endpoint
                                  << " failed: " << write_result.error();
+                    result = tl::unexpected(write_result.error());
                     continue;  // write failed, attempt next candidate
                 } else {
                     // ErrorCode::REPLICA_NUM_EXCEEDED or
@@ -505,9 +506,10 @@ tl::expected<void, ErrorCode> P2PClientService::Put(const ObjectKey& key,
             result.error() != ErrorCode::REPLICA_ALREADY_EXISTS) {
             LOG(ERROR) << "Failed to put key: " << key
                        << " error: " << result.error();
-        } else {
-            // the key exists, just ignore the error
+            return result;
         }
+        // REPLICA_NUM_EXCEEDED / REPLICA_ALREADY_EXISTS: object already
+        // stored, treat as success so callers don't retry needlessly.
     }
 
     return {};