Add functional tests for non-jar exclude and custom r8 args

[Goooler]

Refs #2293.

[Copilot]

🟡 Changes recommended

The new passCustomR8Args functional test’s expectations don’t match Shadow’s default R8 behavior and are likely incorrect/flaky without adjustment.

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Pull request overview

Adds functional-test coverage for two edge cases in Shadow’s functional test suite: excluding a transitive dependency that has no JAR artifact (POM-only), and configuring custom R8 command-line arguments during minimization.

Changes:

• Extend the functional-test Maven repo builder to publish POM-only modules (pomModule / PomModule).
• Add a Filtering functional test that excludes a transitive POM-only dependency.
• Add an R8 minimization functional test intended to validate custom R8 args handling (currently with an assertion that should be adjusted; see review comment).

File summaries

File	Description
src/functionalTest/kotlin/com/github/jengelman/gradle/plugins/shadow/util/AppendableMavenRepository.kt	Adds support for publishing POM-only modules in the test Maven repository.
src/functionalTest/kotlin/com/github/jengelman/gradle/plugins/shadow/BasePluginTest.kt	Extends the shared local Maven repo fixture with a module that depends on a POM-only dependency.
src/functionalTest/kotlin/com/github/jengelman/gradle/plugins/shadow/FilteringTest.kt	Adds a test that applies an exclude rule targeting a transitive POM-only dependency.
src/functionalTest/kotlin/com/github/jengelman/gradle/plugins/shadow/R8MinimizationTest.kt	Adds a test for custom R8 args (needs a deterministic assertion to avoid flakiness).

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 1
• Review effort level: Lite

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