fix: fix self-loop in workflow

[ZenWayne]

Parent node fetching logic to avoid errors caused by self-referencing edges. New test cases added to verify graph cyclic behavior and termination conditions. Confirmed message order is correct and meets expectations

Why are these changes needed?

self looping in workflow not working

remote self from parent node in Digraph

Related issue number

#6676

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[ZenWayne]

@microsoft-github-policy-service agree

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.67%. Comparing base (8c1236d) to head (badeb85).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6677   +/-   ##
=======================================
  Coverage   79.67%   79.67%           
=======================================
  Files         231      231           
  Lines       17172    17173    +1     
=======================================
+ Hits        13682    13683    +1     
  Misses       3490     3490           

Flag	Coverage Δ
unittests	79.67% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 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.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[ekzhu]

Thanks for the PR! Could you run the formatter following instructions in python/README.md

[ZenWayne]

Sure, I have formatted with poe