HADOOP-19902. [ABFS] Fix small write hflush followed by close

[sunchao]

Why are the changes needed?

With fs.azure.write.enableappendwithflush=true, ABFS's small-write optimized hflush() submits and consumes the active DataBlock but leaves it recorded as the active writable block. A later close() retries that already consumed block and fails with IllegalStateException: Expected stream state Writing -but actual state is Closed.

JIRA: HADOOP-19902

What changes were proposed in this PR?

Clear the active block after the optimized append-with-flush path submits it for upload, matching the lifecycle already used by uploadCurrentBlock().

Add a regression test with small-write optimization enabled that executes write(), hflush(), and close(), then asserts the payload is appended exactly once using FLUSH_MODE.

Contains content generated by Codex.

How was this PR tested?

• Unit test: ./mvnw -pl hadoop-tools/hadoop-azure -am -DskipITs -Dtest=org.apache.hadoop.fs.azurebfs.services.TestAbfsOutputStream#verifySmallWriteOptimizedHFlushFollowedByClose -DfailIfNoTests=false test

AI Tooling

• The PR includes the phrase Contains content generated by Codex.
• My use of AI tooling follows ASF legal policy.

[hadoop-yetus]

🎊 +1 overall

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+1 💚	test4tests	0m 0s		The patch appears to include 1 new or modified test files.
			_ trunk Compile Tests _
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+1 💚	mvninstall	0m 34s		the patch passed
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+1 💚	javac	0m 34s		the patch passed
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+1 💚	unit	2m 12s		hadoop-azure in the patch passed.
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		129m 19s

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Docker	ClientAPI=1.54 ServerAPI=1.54 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-8513/1/artifact/out/Dockerfile
GITHUB PR	#8513
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modules	C: hadoop-tools/hadoop-azure U: hadoop-tools/hadoop-azure
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[sunchao]

cc @anmolanmol1234 @anujmodi2021 @steveloughran, could you review this?

It addresses a failure in the small-write append-with-flush path: after hflush() consumes the active block, a subsequent close() can attempt to reuse that already closed block. The PR adds a focused write -> hflush -> close regression test.

[sunchao]

we encountered this while using Parquet 1.15.2 without this PR apache/parquet-java#3204