fix: Move byte-buddy to testImplementation scope

[satsukies]

Closes #281

Summary

net.bytebuddy:byte-buddy was declared with implementation scope, but it is only needed by Spock for mocking concrete classes at test time (it is never referenced under src/). This incorrect scope put byte-buddy on the plugin runtime classpath, so Gradle TestKit injected and repackaged it during acceptance tests — and dependabot's bump to 1.18.9 broke that repackaging on older Gradle (≤ 8.2), failing all acceptance tests with Failed to create Jar file byte-buddy-1.18.9.jar.

See #281 for the full root-cause analysis and history.

Changes

• Move net.bytebuddy:byte-buddy from implementation to testImplementation, matching the neighboring objenesis declaration.
• Apply the chore(deps): bump net.bytebuddy:byte-buddy from 1.15.11 to 1.18.9 #279-equivalent deps update (bump byte-buddy 1.15.11 → 1.18.9) to verify the scope fix resolves the breakage on older Gradle via CI.

Verification

• Confirmed byte-buddy no longer appears in createClasspathManifest / sourceSets.main.runtimeClasspath (the TestKit-injected plugin classpath), so the jar-repackaging failure can no longer occur on any Gradle version.
• Confirmed byte-buddy is removed from runtimeClasspath but retained on testRuntimeClasspath (Spock concrete-class mocking preserved).
• The remaining acceptance-test verification (older Gradle versions) is delegated to CI.

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request updates the net.bytebuddy:byte-buddy dependency in build.gradle from version 1.15.11 to 1.18.9 and changes its configuration from implementation to testImplementation. There are no review comments, and I have no feedback to provide.

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[devin-ai-integration]

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[tnj]

LGTM!

[satsukies]

thanks!