Fix: Stabilize ExtensionLoaderTest.testInjectExtension

[Anshul-creator]

What is the purpose of the change?

This PR fixes order-dependent flakiness (OD-Vic) in ExtensionLoaderTest.testInjectExtension

This test fails under randomized test execution orders due to multiple DemoImpl instances being registered in the shared ScopeBeanFactory of ApplicationModel.

Root Cause

testInjectExtension and related tests (testGetOrDefaultExtension) both interacted with the same shared ScopeBeanFactory.
Because bean registration was not idempotent, repeated or parallel test execution sometimes resulted in duplicate DemoImpl entries, leading to inconsistent injection results or assertion failures.

What's Changed

• Added defensive logic to check existing DemoImpl beans before registering a new one.
• Made registration idempotent by registering only one instance (demo-test-singleton) if none exists.
• Ensured subsequent tests reuse the same canonical bean reference.
• Updated testGetOrDefaultExtension to follow the same consistent setup.

These changes eliminate shared-state interference and make the test deterministic under any order or random seed.

Verification

You can try running the loop below on pre-fix and post-fix code:

fail=0; total=0
for s in 7138 7351 7892 11170 15857 15869 17721 20421 21699 22631 24059 29905; do
  echo "SEED=$s"
  total=$((total+1))
  SEED=$s ./mvnw -pl dubbo-common -DfailIfNoTests=true -DforkCount=1 -DreuseForks=false \
    -Dsurefire.runOrder=random \
    -Djunit.jupiter.testclass.order.default=org.junit.jupiter.api.ClassOrderer\$Random \
    -Djunit.jupiter.testmethod.order.default=org.junit.jupiter.api.MethodOrderer\$Random \
    -Djunit.jupiter.testclass.order.random.seed="$s" \
    -Djunit.jupiter.testmethod.order.random.seed="$s" \
    -Dtest=org.apache.dubbo.common.extension.ExtensionLoaderTest test \
    >/dev/null || { echo "** FAILED on SEED=$s **"; fail=$((fail+1)); }
done
echo
echo "===== SUMMARY ====="
echo "Total seeds: $total"
echo "Failed:      $fail"
echo "Passed:      $((total - fail))"
echo "==================="

The seeds were found by using a script I developed locally. Sometimes not all seeds fail on execution, but some will always fail on pre-fix code. However, on post-fix code none of the seeds fail.

Checklist

• Make sure there is a GitHub_issue field for the change.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Write necessary unit-test to verify your logic correction. If the new feature or significant change is committed, please remember to add sample in dubbo samples project.
• Make sure gitHub actions can pass. Why the workflow is failing and how to fix it?

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.75%. Comparing base (aed93d5) to head (2ba50b1).

Additional details and impacted files

@@             Coverage Diff              @@
##                3.3   #15744      +/-   ##
============================================
  Coverage     60.75%   60.75%              
- Complexity       15    11719   +11704     
============================================
  Files          1938     1938              
  Lines         88627    88627              
  Branches      13381    13380       -1     
============================================
+ Hits          53842    53844       +2     
+ Misses        29255    29244      -11     
- Partials       5530     5539       +9     

Flag	Coverage Δ
integration-tests-java21	32.31% <ø> (+<0.01%)	⬆️
integration-tests-java8	32.53% <ø> (+<0.01%)	⬆️
samples-tests-java21	32.04% <ø> (-0.02%)	⬇️
samples-tests-java8	29.82% <ø> (+0.01%)	⬆️
unit-tests-java11	59.02% <ø> (-0.03%)	⬇️
unit-tests-java17	58.51% <ø> (-0.02%)	⬇️
unit-tests-java21	58.50% <ø> (-0.03%)	⬇️
unit-tests-java8	59.01% <ø> (+0.01%)	⬆️

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