Fix ConcurrentModificationException in CompletionProvider listener iteration

[Copilot]

CompletionProvider.notifyCompletionResolved iterates completionListeners on the completion job worker thread, while addCompletionListener / removeCompletionListener can mutate the set from other threads. Backing it with a plain LinkedHashSet produced the ConcurrentModificationException reported in the issue:

java.util.ConcurrentModificationException
    at java.base/java.util.LinkedHashMap$LinkedHashIterator.nextNode(...)
    at CompletionProvider$CompletionJob.notifyCompletionResolved(CompletionProvider.java:289)
    at CompletionProvider$CompletionJob.run(CompletionProvider.java:170)

Changes

• CompletionProvider.java: switch completionListeners from LinkedHashSet to CopyOnWriteArraySet. Iteration is snapshot-based and safe under concurrent add/remove, which fits a read-heavy listener collection and aligns with the project's convention of using copy-on-write collections for concurrent listener access.

// before
this.completionListeners = new LinkedHashSet<>();
// after
this.completionListeners = new CopyOnWriteArraySet<>();

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• checkstyle.org
  • Triggering command: /opt/hostedtoolcache/CodeQL/2.25.1/x64/codeql/tools/linux64/java/bin/java /opt/hostedtoolcache/CodeQL/2.25.1/x64/codeql/tools/linux64/java/bin/java -jar /opt/hostedtoolcache/CodeQL/2.25.1/x64/codeql/xml/tools/xml-extractor.jar --fileList=/tmp/codeql-scratch-8a343ad76595886d/dbs/java/working/files-to-index11279688604168215133.list --sourceArchiveDir=/tmp/codeql-scratch-8a343ad76595886d/dbs/java/src --outputDir=/tmp/codeql-scratch-8a343ad76595886d/dbs/java/trap/java (dns block)
• www.eclipse.org
  • Triggering command: /usr/lib/jvm/temurin-17-jdk-amd64/bin/java /usr/lib/jvm/temurin-17-jdk-amd64/bin/java -classpath /home/REDACTED/.m2/wrapper/dists/apache-maven-3.9.9/3477a4f1/boot/plexus-classworlds-2.8.0.jar -Dclassworlds.conf=/home/REDACTED/.m2/wrapper/dists/apache-maven-3.9.9/3477a4f1/bin/m2.conf -Dmaven.home=/home/REDACTED/.m2/wrapper/dists/apache-maven-3.9.9/3477a4f1 -Dlibrary.jansi.path=/home/REDACTED/.m2/wrapper/dists/apache-maven-3.9.9/3477a4f1/lib/jansi-native -Dmaven.multiModuleProjectDirectory=/home/REDACTED/work/copilot-for-eclipse/copilot-for-eclipse org.codehaus.plexus.classworlds.launcher.Launcher -pl com.microsoft.copilot.eclipse.core -am compile -q (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

fixes microsoft/copilot-eclipse-feedback#181

[ethanyhou]

LGTM