feat(activemq): add execArtemisCommand helper to ArtemisContainer

[glasses-and-hat]

Summary

Closes #11589

The ArtemisContainer bundles the Artemis CLI at /var/lib/artemis-instance/bin/artemis, but using it from tests requires boilerplate: hardcoding the binary path and manually appending --user / --password on every call. This PR adds a convenience method that hides those details:

var result = artemis.execArtemisCommand(
    "queue", "create", "--name=my-queue", "--auto-create-address", "--anycast", "--silent"
);
assertThat(result.getExitCode()).isEqualTo(0);

Changes:

• ArtemisContainer.execArtemisCommand(String... commands) — prepends the CLI binary path and appends --user / --password from the container's configured credentials, then delegates to execInContainer.
• New test ArtemisContainerTest#execArtemisCommand that creates an AMQP queue via the CLI and asserts a zero exit code.
• Docs updated with a usage snippet for execArtemisCommand.

Test plan

• ./gradlew :testcontainers-activemq:spotlessApply passes with no changes
• ./gradlew :testcontainers-activemq:test — requires Docker; verified locally that the container starts and the queue creation command exits 0