HDFS-17891 HostSet.add() throws uncaught exception if DataNode hostname is not resolved

[Jimvin]

Description of PR

This patch changes the hostname resolution check in HostSet.add() to skip unresolved hosts rather than throwing an IllegalArgumentException. It fixes hdfs dfsadmin -report where one or more datanode hostnames are unresolvable.

For code changes:

• Does the title or this PR starts with the corresponding JIRA issue id (e.g. 'HADOOP-17799. Your PR title ...')?

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[hadoop-yetus]

💔 -1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	1m 18s		Docker mode activated.
			_ Prechecks _
+1 💚	dupname	0m 0s		No case conflicting files found.
+0 🆗	codespell	0m 0s		codespell was not available.
+0 🆗	detsecrets	0m 0s		detect-secrets was not available.
+1 💚	@author	0m 0s		The patch does not contain any @author tags.
-1 ❌	test4tests	0m 0s		The patch doesn't appear to include any new or modified tests. Please justify why no new tests are needed for this patch. Also please list what manual steps were performed to verify this patch.
			_ trunk Compile Tests _
+1 💚	mvninstall	41m 50s		trunk passed
+1 💚	compile	1m 45s		trunk passed with JDK Ubuntu-21.0.10+7-Ubuntu-124.04
+1 💚	compile	1m 45s		trunk passed with JDK Ubuntu-17.0.18+8-Ubuntu-124.04.1
+1 💚	checkstyle	1m 47s		trunk passed
+1 💚	mvnsite	1m 54s		trunk passed
+1 💚	javadoc	1m 27s		trunk passed with JDK Ubuntu-21.0.10+7-Ubuntu-124.04
+1 💚	javadoc	1m 28s		trunk passed with JDK Ubuntu-17.0.18+8-Ubuntu-124.04.1
+1 💚	spotbugs	4m 3s		trunk passed
+1 💚	shadedclient	31m 0s		branch has no errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+1 💚	mvninstall	1m 22s		the patch passed
+1 💚	compile	1m 15s		the patch passed with JDK Ubuntu-21.0.10+7-Ubuntu-124.04
+1 💚	javac	1m 15s		the patch passed
+1 💚	compile	1m 18s		the patch passed with JDK Ubuntu-17.0.18+8-Ubuntu-124.04.1
+1 💚	javac	1m 18s		the patch passed
+1 💚	blanks	0m 0s		The patch has no blanks issues.
-0 ⚠️	checkstyle	1m 15s	/results-checkstyle-hadoop-hdfs-project_hadoop-hdfs.txt	hadoop-hdfs-project/hadoop-hdfs: The patch generated 1 new + 0 unchanged - 0 fixed = 1 total (was 0)
+1 💚	mvnsite	1m 24s		the patch passed
+1 💚	javadoc	0m 57s		the patch passed with JDK Ubuntu-21.0.10+7-Ubuntu-124.04
+1 💚	javadoc	0m 59s		the patch passed with JDK Ubuntu-17.0.18+8-Ubuntu-124.04.1
+1 💚	spotbugs	3m 45s		the patch passed
+1 💚	shadedclient	30m 8s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
-1 ❌	unit	275m 29s	/patch-unit-hadoop-hdfs-project_hadoop-hdfs.txt	hadoop-hdfs in the patch passed.
+1 💚	asflicense	0m 46s		The patch does not generate ASF License warnings.
		405m 44s

Reason	Tests
Failed junit tests	hadoop.hdfs.server.balancer.TestBalancerWithHANameNodes
	hadoop.hdfs.server.namenode.ha.TestStandbyCheckpoints

Subsystem	Report/Notes
Docker	ClientAPI=1.54 ServerAPI=1.54 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-8341/1/artifact/out/Dockerfile
GITHUB PR	#8341
Optional Tests	dupname asflicense compile javac javadoc mvninstall mvnsite unit shadedclient spotbugs checkstyle codespell detsecrets
uname	Linux c82cb9217f53 5.15.0-164-generic #174-Ubuntu SMP Fri Nov 14 20:25:16 UTC 2025 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	dev-support/bin/hadoop.sh
git revision	trunk / 5082f4f
Default Java	Ubuntu-17.0.18+8-Ubuntu-124.04.1
Multi-JDK versions	/usr/lib/jvm/java-21-openjdk-amd64:Ubuntu-21.0.10+7-Ubuntu-124.04 /usr/lib/jvm/java-17-openjdk-amd64:Ubuntu-17.0.18+8-Ubuntu-124.04.1
Test Results	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-8341/1/testReport/
Max. process+thread count	3021 (vs. ulimit of 5500)
modules	C: hadoop-hdfs-project/hadoop-hdfs U: hadoop-hdfs-project/hadoop-hdfs
Console output	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-8341/1/console
versions	git=2.43.0 maven=3.9.11 spotbugs=4.9.7
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