Isolate Kubernetes plugin test working directories under target/

[gnodet]

Summary

Kubernetes plugin tests (KubernetesRunTest, KubernetesRunCustomTest) create
.camel-jbang-run/<project>/ directories in the module root via KubernetesRun.doCall().
These directories are never cleaned up, which causes two problems:

1. Concurrent test collisions: both test classes write to the same project directories
   (e.g. .camel-jbang-run/route/, .camel-jbang-run/my-route-props/), so they cannot
   run in parallel.
2. Stale files in the source tree: leftover directories can cause ENOENT race conditions
   when docs gulp scans the source tree with ** globs.

Changes

• Extract a getRunPlatformDir() method in KubernetesRun (default: .camel-jbang-run)
  that tests override to target/tests/<ClassName> via anonymous subclass. This isolates
  each test class into its own directory under target/, which is gitignored and cleaned
  by mvn clean.
• Add @AfterAll/@AfterEach cleanup in the test classes to remove the per-class directory.
• Add missing CommandLineHelper.useHomeDir("target") to KubernetesRunCustomTest to
  prevent .camel-jbang/ leaking into the source tree.

Test plan

• mvn test -B -pl dsl/camel-jbang/camel-jbang-plugin-kubernetes — all 128 tests pass
• No leftover .camel-jbang-run/ directories after test run
• Formatting clean (mvn formatter:format impsort:sort)

[github-actions]

🌟 Thank you for your contribution to the Apache Camel project! 🌟
🤖 CI automation will test this PR automatically.

🐫 Apache Camel Committers, please review the following items:

• First-time contributors require MANUAL approval for the GitHub Actions to run
• You can use the command /component-test (camel-)component-name1 (camel-)component-name2.. to request a test from the test bot although they are normally detected and executed by CI.
• You can label PRs using build-all, build-dependents, skip-tests and test-dependents to fine-tune the checks executed by this PR.
• Build and test logs are available in the summary page. Only Apache Camel committers have access to the summary.

⚠️ Be careful when sharing logs. Review their contents before sharing them publicly.