Improve ContainerSpecHelper partial image configuration error message

[venkateshwaracholan]

Summary

Fixes #4806

Improve the error message shown when only one of the container image name or image tag is configured.

Changes

• Reword the IllegalArgumentException message to improve clarity.
• Remove awkward wording and the double-space in the message.
• Clarify that both image name and image tag must be specified together, or both omitted to use the default image configuration.
• Add tests covering valid and invalid image configuration combinations.

Checklist

• 🛡️ Don't disclose security issues! (contact security@apache.org)
• 🔗 Clearly explained why the changes are needed, or linked related issues: Fixes #
• 🧪 Added/updated tests with good coverage, or manually tested (and explained how)
• 💡 Added comments for complex logic
• 🧾 Updated CHANGELOG.md (if needed)
• 📚 Updated documentation in site/content/in-dev/unreleased (if needed)

[dimas-b]

The new message change looks better 🙂 Thanks, @venkateshwaracholan !

[dimas-b]

IMHO, using System properties during unit tests is not nice and prone to inter-test side effects.

I support the idea of adding test coverage to the image selection logic, though. I'd propose to add a test-only variant for containerSpecHelper() that would take Function<String, String> for both the sys. prop. and env. lookups (overridden by tests). The "prod" code will use trivial redirects on the main call path.

This way we'd be able to validate env. lookup logic too.

[venkateshwaracholan]

@dimas-b Thanks for the suggestion. I updated the tests to inject property and environment lookups instead of modifying JVM system properties directly. The production path is unchanged, and the new approach also allowed adding coverage for environment variable handling and precedence rules.