getShards returns 500 "Wrong constructor found" for valid masterchain seqno

[lee20260601]

Calling GET /api/v2/getShards?seqno=<valid_masterchain_seqno> consistently
returns HTTP 500 with error "INTERNAL: get last block failed Wrong constructor
found(during last block synchronization)" for any valid, already-finalized
masterchain seqno.

Steps to reproduce:

1. curl http://localhost:8081/api/v2/getMasterchainInfo (get current seqno, e.g. 84314454)
2. curl "http://localhost:8081/api/v2/getShards?seqno=84314454"
   -> 500 error

Tested across multiple seqnos (spanning 450+ blocks), all fail with the same error.
getMasterchainInfo and getConsensusBlock work correctly on the same deployment,
confirming liteserver connectivity is healthy.

The equivalent endpoint on the Python version (toncenter/ton-http-api) using
the same mainnet.json liteserver config, GET /shards?seqno=84314400, returns
correct data — ruling out a liteserver/network issue.

Environment:

• Image: toncenter/ton-http-api-cpp:v2.1.6
• config: default config_vars.yaml, tonlib_config_path pointed to standard mainnet global config