[Audit] Fix password audit Sonar issues

[Caideyipi]

Description

Follow-up to #18357.

• Compare tree-model usernames null-safely.
• Replace the password-redaction regex with a simple prefix matcher and linear quoted-literal scan.
• Use pattern variables for password-change statement extraction.
• Stop catching Error in the audit task wrapper.
• Merge equivalent privilege switch labels and make early audit fields final.
• Document the intentionally exhaustive relational authorization switch instead of high-risk restructuring.
• Suppress the unavoidable generated-i18n line-length warning in the tree authorization helper.

Tests

• mvn -pl iotdb-core/datanode -Dtest=DNAuditLoggerTest,PasswordChangeAuditContextTest -Ddevelocity.off=true test (18 tests)
• mvn -pl iotdb-core/datanode -Dtest=DNAuditLoggerTest -Ddevelocity.off=true test (5 tests, final redaction implementation)
• DataNode compile, Checkstyle, and Spotless checks included in the Maven runs
• git diff --check