fix(chain): return TxNode from CanonicalIter

[shubham-shinde-442]

Fixes: #2051

Description

This patch implements a TODO comment to have CanonicalIter return TxNode directly instead of just (Txid, Arc). This eliminates a redundant lookup in CanonicalView::new that was fetching the same transaction node data twice.

• Modified CanonicalIter to return TxNode<'g, Arc, A> instead of (Txid, Arc) in its iterator item type
• Simplified CanonicalView::new by removing redundant tx_graph.get_tx_node(txid) lookup and TODO comment
• Added integration test test_canonical_view_construction_with_tx_nodes to verify the change

Changelog notice

All Submissions:

• I followed the contribution guidelines

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[evanlinjin]

Thanks for the attempt, but this is no longer needed as we will remove CanonicalIter in #2038.