Add more docs and tests for DuplicatesStrategy

[Goooler]

• Follow up Note the mixed usages of ResourceTransformer and DuplicatesStrategy #1349.
• Closes Exclude a specific class from a dependency but it's the same name as a local one #1357.

[Copilot]

Pull Request Overview

This PR enhances documentation and test coverage for the DuplicatesStrategy functionality in ShadowJar tasks, following up on issue #1349. The changes improve clarity around how duplicate file handling works and provide comprehensive examples.

• Adds detailed documentation for the getDuplicatesStrategy() method with examples of each strategy type
• Adds functional tests demonstrating how to override the duplicates strategy using eachFile and filesMatching
• Updates existing documentation to clarify interactions between ResourceTransformers and duplicatesStrategy

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
ShadowJar.kt	Adds comprehensive KDoc for getDuplicatesStrategy() method with strategy examples and transformer interaction notes
ServiceFileTransformerTest.kt	Adds tests for overriding EXCLUDE strategy using eachFile and filesMatching, fixes line separator handling
ShadowJarCachingTest.kt	Fixes test setup by using writeText instead of appendText to prevent accumulating configurations
merging/README.md	Enhances documentation with clearer examples of strategy override techniques and additional reference links
shadow.api	Updates API surface to include the newly documented getDuplicatesStrategy() method