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?