Fix advance green-edge detection when parent is behind remote

[PawelLipski]

Summary

• Extract shared is_connected_with_green_edge() on MacheteClient so advance and status use the same green-edge criteria.
• Fixes a leftover from 3.41.0: when the parent branch is behind its remote counterpart and a child was forked from the remote tip, status already renders a green edge but advance only considered children whose fork point equals the parent's local tip.
• Add regression tests for the parent-behind-remote scenario (single and multiple green-edge children).

Test plan

• tox -e py -- tests/test_advance.py
• tox -e py -- tests/test_advance.py tests/test_status.py -k "advance_when_parent_behind_remote or no_yellow_edge_when_parent_behind_remote"
• tox -e mypy
• Verified manually in rules_scala: advance -y on master now sees both green-edge children (ambiguous advance) instead of only the one whose fork point equals local master.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 81.81818% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.67%. Comparing base (1c007c1) to head (d12c173).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
git_machete/client/base.py	78.94%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1751      +/-   ##
===========================================
- Coverage    98.77%   98.67%   -0.10%     
===========================================
  Files           45       45              
  Lines         5372     5378       +6     
  Branches       981      983       +2     
===========================================
+ Hits          5306     5307       +1     
- Misses          41       44       +3     
- Partials        25       27       +2     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.