Enable Gradle configuration cache

[ogolberg]

Summary

• Make ExpediterTask configuration-cache compatible: report → RegularFileProperty, new @Input projectName: Property<String> (replaces project.name access at execution time), and the mutable List<ArtifactCollection> fields are replaced by a ConfigurableFileCollection for tracked inputs plus a ListProperty<ClassfileSource> populated lazily via a new ArtifactCollection.toClassfileSources() provider helper.
• Bump testkit-plugin to 0.0.18 for the InstrumentWithJacocoOfflineTask config-cache fix from testkit-plugins#42 (0.0.17's publish workflow failed; 0.0.18 is the re-release). Add a root-project workaround for a latent CC bug in testkit's publishOnlyIf — its predicate reads PublishToMavenRepository.publication at execution time, which Gradle discards under CC; we replace the onlyIf spec with one derived from the task name.
• Turn on org.gradle.configuration-cache=true in the project gradle.properties and add the same to each plugin/src/test/projects/ExpediterPluginIntegrationTest/* fixture so the plugin is exercised under CC end-to-end.

Test plan

• ./gradlew build passes with Configuration cache entry stored on first run and Configuration cache entry reused on the next.
• ./gradlew :plugin:test passes all 44 integration tests with CC enabled in both the outer build and the fixtures (JDK 21 — older Gradle versions in the test matrix don't support JDK 25).
• Re-run CI to confirm.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 91.30435% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.94%. Comparing base (6ffd213) to head (6355997).

Files with missing lines	Patch %	Lines
...lin/com/toasttab/expediter/gradle/ExpediterTask.kt	88.23%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #259   +/-   ##
=========================================
  Coverage     90.94%   90.94%           
+ Complexity      340      337    -3     
=========================================
  Files            44       44           
  Lines          1137     1137           
  Branches        157      157           
=========================================
  Hits           1034     1034           
  Misses           52       52           
  Partials         51       51           

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