KAFKA-19913 Add comprehensive unit tests for VerifiableProperties class

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This PR adds comprehensive unit test coverage for the VerifiableProperties class, which previously had no dedicated test file.

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Test Plan

% ./gradlew :core:test --tests kafka.utils.VerifiablePropertiesTest

> Configure project :
Starting build with version 4.2.0-SNAPSHOT (commit id aa3a5359) using Gradle 9.1.0, Java 17 and Scala 2.13.17
Build properties: ignoreFailures=false, maxParallelForks=10, maxScalacThreads=8, maxTestRetries=0

> Task :core:test

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetBooleanWithDefaultThrowsOnInvalidValue() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testConstructorWithProperties() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetStringWithDefault() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetMapWithComplexValues() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetShortWithDefaultAcceptsMinMax() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetShortWithDefault() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetLongRequired() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetIntRequired() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testToString() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetBooleanWithDefault() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetIntNegativeValue() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetBooleanInvalidFormatThrowsException() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetLongWithDefault() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetStringRequiredThrowsWhenMissing() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetLongWithDefaultAcceptsMinMax() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testContainsKey() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testPropertiesReferenceSetTracking() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetMapWithValidEntries() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testCompanionObjectApply() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetPropertyEmptyString() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetPropertyTrimsWhitespace() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetIntWithDefault() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetIntInvalidFormatThrowsException() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetStringRequired() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetDoubleRequired() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetDoubleRequiredThrowsWhenMissing() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetDoubleInvalidFormatThrowsException() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetMapWithValidationFunction() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetLongInvalidFormatThrowsException() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetMapWithEmptyString() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetIntRequiredThrowsWhenMissing() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetShortInvalidFormatThrowsException() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetDoubleNegativeValue() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetMapWithMalformedEntries() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetBooleanRequiredThrowsWhenMissing() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetPropertyReturnsNull() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetStringEmptyString() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetLongNegativeValue() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetLongRequiredThrowsWhenMissing() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetMapWithValidationFunctionThrowsOnInvalidValue() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetDoubleWithDefault() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetMapWithSingleEntry() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetIntWithDefaultAcceptsMinMax() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testGetBooleanRequired() PASSED

Gradle Test Run :core:test > Gradle Test Executor 24 > VerifiablePropertiesTest > testDefaultConstructor() PASSED

Deprecated Gradle features were used in this build, making it incompatible with Gradle 10.

You can use '--warning-mode all' to show the individual deprecation warnings and determine if they come from your own scripts or plugins.

For more on this, please refer to https://docs.gradle.org/9.1.0/userguide/command_line_interface.html#sec:command_line_warnings in the Gradle documentation.

BUILD SUCCESSFUL in 2s
82 actionable tasks: 1 executed, 81 up-to-date
Consider enabling configuration cache to speed up this build: https://docs.gradle.org/9.1.0/userguide/configuration_cache_enabling.html

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Can someone review this please?

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@m1a2st @chia7712 Can anyone of you please review?