Fix flaky InputStreamStreamMessageTest.emptyInputStream()

[ikhoon]

Motivation:

Subscriber.onComplete() is invoked before completionFuture is completed, so a happen-before relationship between byteStreamMessage.isOpen() and Subscriber.onComplete() cannot be guaranteed.

armeria/core/src/main/java/com/linecorp/armeria/common/stream/InputStreamStreamMessage.java

Lines 375 to 376 in 0d94d0d

	downstream.onComplete();
	completionFuture.complete(null);

Modifications:

• Use .whenComplete().join() to explicitly wait until InputStreamStreamMessage completes.

Result:

Closes #6483

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.09%. Comparing base (8150425) to head (79a1be5).
⚠️ Report is 216 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #6484      +/-   ##
============================================
- Coverage     74.46%   74.09%   -0.37%     
- Complexity    22234    22991     +757     
============================================
  Files          1963     2062      +99     
  Lines         82437    86128    +3691     
  Branches      10764    11310     +546     
============================================
+ Hits          61385    63817    +2432     
- Misses        15918    16893     +975     
- Partials       5134     5418     +284     

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[github-actions]

🔍 Build Scan® (commit: 79a1be5)

Job name	Status	Build Scan®
build-ubicloud-standard-16-jdk-8	✅	https://ge.armeria.dev/s/e6kmx5q2lb5rg
build-ubicloud-standard-16-jdk-21-snapshot-blockhound	❌ (failure)	https://ge.armeria.dev/s/fpfbri7tya5vk
build-ubicloud-standard-16-jdk-17-min-java-17-coverage	✅	https://ge.armeria.dev/s/emtvp7wql4ytu
build-ubicloud-standard-16-jdk-17-min-java-11	✅	https://ge.armeria.dev/s/p3asbgzlavqtu
build-ubicloud-standard-16-jdk-17-leak	✅	https://ge.armeria.dev/s/cujxmcepmzrmo
build-ubicloud-standard-16-jdk-11	✅	https://ge.armeria.dev/s/rnrk3ebjurvpc
build-macos-latest-jdk-21	✅	https://ge.armeria.dev/s/ludirvcn3i3rw

[minwoox]

👍