Writes to a non-main branch uses main's head as the parent snapshot, corrupting branch lineage

[badalprasadsingh]

Apache Iceberg version

main (development)

Please describe the bug 🐞

Any write operation (Append, AppendTable, Overwrite, or Delete) executed through a transaction created with NewTransactionOnBranch(branch) for a branch other than main builds the new snapshot with the wrong parent.

Impact:

• The new snapshot records the main branch's head as its parent instead of the target branch's head.
• The branch silently inherits data files from main, leading to incorrect snapshot lineage and data corruption.

Contribution:
I'll submit a PR to fix this issue.