Support op_seek_blob operation in Firebird Proxy

# Feature Request

**Is your feature request related to a problem?**

Firebird Proxy does not support the op_seek_blob operation.

**Describe the feature you would like.**

Implement op_seek_blob in Firebird Proxy: reposition the read offset of an opened BLOB according to the requested seek mode and offset, and return the resulting position in the response (as fbclient and Jaybird expect). Seek is only valid for stream BLOBs, matching Firebird server behavior.

Start by locating the Firebird Proxy implementation and its handling for existing BLOB operations. Read how opened stream BLOBs process seek modes and offsets, then verify that op_seek_blob returns the resulting position and rejects non-stream BLOBs consistently with the stated server behavior.

Written by the indexing model from the issue text.