Add functional tests for PropertiesFileTransformer

[Goooler]

Refs

• Introduce CleanProperties #622
• PropertiesFileTransformer breaks Reproducible builds in 8.1.1 #856
• Fix for PropertiesFileTransformer breaks Reproducible builds in 8.1.1 #856 #858
• Fix for PropertiesFileTransformer breaks Reproducible builds in 8.1.1 Goooler/shadow#102

[Copilot]

Pull Request Overview

Adds functional tests for the PropertiesFileTransformer, ensuring various merge strategies, key transformations, charset handling, mappings, and removal of the default timestamp header.

• Documented CleanProperties behavior with KDoc
• Removed the generic PropertiesFileTransformer entry from the shared transformer matrix
• Added a new PropertiesFileTransformerTest suite covering multiple scenarios

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/main/kotlin/com/github/jengelman/gradle/plugins/shadow/internal/CleanProperties.kt	Added KDoc explaining suppression of the timestamp header
src/functionalTest/kotlin/com/github/jengelman/gradle/plugins/shadow/transformers/TransformersTest.kt	Removed the PropertiesFileTransformer case from the matrix
src/functionalTest/kotlin/com/github/jengelman/gradle/plugins/shadow/transformers/PropertiesFileTransformerTest.kt	Introduced functional tests for PropertiesFileTransformer

Comments suppressed due to low confidence (2)

src/main/kotlin/com/github/jengelman/gradle/plugins/shadow/internal/CleanProperties.kt:10

• [nitpick] The KDoc summary is a bit vague; consider clarifying that this class overrides store to suppress the default date comment header, for example: "CleanProperties overrides store to prevent writing the timestamp comment."

 * Introduced in order to remove prepended timestamp when creating output stream.

src/functionalTest/kotlin/com/github/jengelman/gradle/plugins/shadow/transformers/TransformersTest.kt:197

• Removing the PropertiesFileTransformer entry from the generic transformer matrix may leave its core behavior (especially keyTransformer logic) untested at that level; ensure these scenarios are fully covered by the new functional tests.

      "" to ManifestResourceTransformer::class,