build: enforce existing Checkstyle rules

[ZYZ666-RGB]

Description

Checkstyle currently reports violations without failing validation. Enable checkstyle.failOnViolation=true and clean up the existing trailing whitespace and unused imports so the current rules can be enforced.

The Java changes are limited to whitespace and unused imports, including checked-in protobuf/gRPC sources. The Checkstyle rules and source coverage remain unchanged.

Validation

• Checkstyle 9.3 with the repository configuration: all 1,132 tracked Java files pass, with zero violations.
• mvn -o -pl spec validate passes after removing the temporary probes.
• Four isolated negative checks confirm that trailing whitespace and unused imports each fail Maven validation in both production and test sources. No probe files are included in the commit.
• git diff --check passes with CRLF handling enabled; the existing CRLF file is preserved.

The previous October 8 verification passed validation across all 83 reactor modules. The full test run reported four failures and one error in OpenTelemetry tests; the same cases were reproduced using unmodified baseline sources. Docker-dependent tests were skipped. A fresh full build on October 9 did not complete in the recreated environment, so a fully green test suite is not claimed; repository CI should provide that confirmation.

• Followed the CONTRIBUTING Guide.
• Used a Conventional Commit PR title.
• Ensure the full test suite passes (limitations described above).
• Appropriate READMEs updated if necessary (not applicable to this change).

Fixes #1206