fix(#16): fail render when custom plugin has no class

[robfrank]

Closes #16

Summary

When a custom plugin was enabled without a class, the arcadedb.plugin.parameters helper rendered -Darcadedb.server.plugins=<name>:%!s() — Go's formatting error for a nil value — producing a silently broken StatefulSet command. This adds a fail-fast guard in _arcadedb.plugin.ports that mirrors the existing missing-port check, so the render now aborts with a clear message identifying the offending plugin.

Test plan

• helm unittest charts/arcadedb — full suite passes (165 tests)
• New case in helpers_test.yaml: custom plugin enabled with a port but no class → failedTemplate with Custom plugin '<name>' has no class specified.
• Existing case: custom plugin with a class still renders -Darcadedb.server.plugins=<name>:<class>
• helm lint charts/arcadedb clean

[gemini-code-assist]

Code Review

This pull request addresses an issue where custom plugins without a specified class render a broken configuration flag. It adds a fail-fast validation check in the Helm chart helpers to ensure custom plugins define a class, includes a corresponding unit test to verify this behavior, and documents the issue and its resolution. Additionally, .worktrees/ has been added to the .gitignore file. There are no review comments to address, and no further feedback is provided.

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