Fix source scan race in plan visitor test

[shuwenwei]

Summary

• Limit IPlanVisitorInheritanceConstraintTest source scanning to discovered src/main/java roots.
• Use walkFileTree to skip target directories while discovering source roots.
• Avoid races with concurrent tests creating or deleting target files.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 40.64%. Comparing base (623532a) to head (34331bd).
⚠️ Report is 6 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #17786      +/-   ##
============================================
- Coverage     40.78%   40.64%   -0.14%     
+ Complexity     2613     2610       -3     
============================================
  Files          5184     5184              
  Lines        350727   350728       +1     
  Branches      44857    44857              
============================================
- Hits         143041   142561     -480     
- Misses       207686   208167     +481     

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