test: add unit tests for Alarm model class

[Titanism0v0]

Summary

• add focused unit coverage for the Alarm status constants and record accessors
• cover open/resolved alarms with different levels and detection ways
• verify resolve() returns a resolved copy without mutating the original alarm
• cover record equality, hash code, and string representation

Why

Alarm is a core alerting model and did not have direct unit coverage. These tests characterize its current public API and immutable resolution behavior, following the existing JUnit 5 and AssertJ test style.

Impact

This is a test-only change. Production behavior is unchanged.

Validation

• mvn -pl ai-apm-web -am -Dskip.frontend=true -Dtest=AlarmTest -Dsurefire.failIfNoSpecifiedTests=false test — BUILD SUCCESS, 6 tests passed
• mvn test — attempted on Windows; ai-apm-common passed, then the reactor stopped on the existing ai-apm-ingest OtlLogLineTest.toJsonBytesTruncatesBodyByConfiguredStringLength source-encoding assertion before reaching ai-apm-web

Closes #29