Enhanced Plugin Command Validation

Better error handling when plugin manifests contain invalid command entries. The system now:

• Validates that array-format commands contain only strings
• Logs descriptive errors for malformed entries: "Unexpected command format in marketplace entry for {plugin-name}"
• Skips invalid entries instead of crashing
• Evidence: Type validation in plugin loader at lines 72627-72633 in loadPluginsFromMarketplace() function