CI: Unit Tests for DAG Module

[d4v1d03]

Add unit tests for sdk/dag.py covering DAGRoot, ChunkDAG, build_dag(), UnixFS node creation, CID operations, varint encoding/decoding, node parsing, and fallback behavior when IPLD is unavailable.

[yashksaini-coder]

can I take on this issue ? @d4v1d03

[d4v1d03]

Sure