test(chain): cover TxGraph::insert_anchor highest-height indexing

[evanlinjin]

Description

Adds a regression test that TxGraph::insert_anchor keeps exactly one txids_by_descending_anchor_height entry per tx, at its highest anchor height, and that the index is rebuilt the same way from the changesets.

The bug in #2344 (the index used the last anchor in the BTreeSet, which wasn't necessarily the highest one) went away with #2299: Anchor now only has anchor_block(), and its Ord compares the BlockId first, so the last anchor is the highest. This test pins that down, using ConfirmationBlockTimes whose confirmation times aren't in height order, plus a second anchor at the same height.

Unlike #2345, this is test-only: the original's code change is no longer in its diff after the rebase, and its CanonicalTask unit test covers the same ordering through a crate-private API. assert_eq!(recovered, graph) already compares the index (TxGraph derives PartialEq), so the test doesn't need a separate check on recovered.

Replaces #2345
Fixes #2344

Changelog notice

None (test only).

Checklists

All Submissions:

• I followed the contribution guidelines

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.71%. Comparing base (fbe1aa6) to head (ab40d6b).
⚠️ Report is 18 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2390      +/-   ##
==========================================
+ Coverage   79.11%   79.71%   +0.60%     
==========================================
  Files          31       31              
  Lines        6061     6148      +87     
  Branches      289      291       +2     
==========================================
+ Hits         4795     4901     +106     
+ Misses       1187     1176      -11     
+ Partials       79       71       -8     

Flag	Coverage Δ
rust	79.71% <ø> (+0.60%)	⬆️

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