Fix BeamRowWrapperTest microsecond precision flake

[bvolpato]

Summary

Fixes #39266.

BeamRowWrapperTest used Instant.now() for a MicrosInstant field. Java 21 can return nanosecond-precision values, while MicrosInstant intentionally rejects precision finer than microseconds. The static row fixture therefore failed to initialize and all 18 tests in the class failed.

Use a deterministic, microsecond-aligned Instant fixture and assert its exact converted value. Production behavior remains unchanged.

Repro command

env JAVA_HOME=$HOME/.sdkman/candidates/java/21.0.6-tem \
  PATH=$HOME/.sdkman/candidates/java/21.0.6-tem/bin:$PATH \
  ./gradlew --no-daemon --console=plain \
  :sdks:java:io:iceberg:test \
  --tests org.apache.beam.sdk.io.iceberg.BeamRowWrapperTest

Before this change: 18 tests failed from the static initializer. After this change: targeted class passes.

Testing

• Java 21 targeted BeamRowWrapperTest: passed
• Java 21 :sdks:java:io:iceberg:build: passed
• :sdks:java:io:iceberg:spotlessCheck: passed

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[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a test flake in BeamRowWrapperTest caused by Java 21's higher-precision clock. By replacing dynamic time generation with a deterministic fixture, the test suite now reliably handles microsecond-precision requirements without being affected by system-level nanosecond resolution.

Highlights

• Test Stability: Replaced the non-deterministic Instant.now() with a fixed, microsecond-aligned Instant fixture to prevent test failures on Java 21.
• Precision Handling: Updated the test assertion to use a hardcoded microsecond value, ensuring compatibility with Java 21's nanosecond-precision timestamps.

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[gemini-code-assist]

Code Review

This pull request refactors BeamRowWrapperTest.java to use a static Instant (TEST_MICROS_INSTANT) instead of Instant.now(), making the microsecond-precision instant logical type conversion test deterministic. The reviewer feedback suggests using Instant.parse with an ISO-8601 string for better readability and removing an unnecessary explicit cast to (long) to avoid potential null-pointer exceptions during unboxing.

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R: @kennknowles for label java.

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[ahmedabu98]

Looks great, thanks for fixing it!