fix: restrict batch import by gateway type and product type compatibility

[lexburner]

📝 Description

• Fix HigressModelResult missing modelApiId/modelApiName fields, which caused the import dialog to show empty service names when importing from Higress
• Add backend validation in validateImportParam: Higress gateway only supports MCP_SERVER batch import; REST_API, AGENT_API, and MODEL_API are explicitly rejected with clear error messages
• Add backend validation: Nacos source does not support REST_API import
• Fix HigressOperator.fetchAgentAPIs() returning null instead of empty PageResult, which could cause NPE
• Frontend: show all source type options (Higress/AI Gateway/Nacos) but disable unsupported combinations with tooltip explaining why, instead of hiding them — this is more friendly for community users
• Frontend: hide "批量导入" button for AGENT_SKILL and WORKER product types, as they have their own dedicated Nacos import flow

Background

The batch import feature was originally designed for MCP_SERVER and AGENT_API from Gateway/Nacos. However, it was inadvertently exposed for all product types including REST_API, MODEL_API, AGENT_SKILL, and WORKER. This caused several issues:

Product Type	Higress	AI Gateway	Nacos
MCP_SERVER	✅ Works	✅ Works	✅ Works
AGENT_API	❌ Returns null → NPE	✅ Works	✅ Works
MODEL_API	❌ Route name ≠ model ID	✅ Works	❌ Not supported
REST_API	❌ Throws UnsupportedOperationException	✅ Works	❌ Not supported
AGENT_SKILL/WORKER	N/A (has dedicated import)	N/A	N/A

🔗 Related Issues

Fix #272

✅ Type of Change

• Bug fix (non-breaking change)
• New feature (non-breaking change)
• Breaking change
• Documentation update
• Code refactoring
• Performance improvement

🧪 Testing

• Manual testing completed
• Verified Higress + REST_API returns INVALID_REQUEST with clear message
• Verified Higress + MODEL_API returns INVALID_REQUEST with clear message
• Verified Higress + AGENT_API returns INVALID_REQUEST with clear message
• Verified Higress + MCP_SERVER still works correctly
• Verified frontend disables unsupported source types with tooltip

📋 Checklist

• Code has been formatted (mvn spotless:apply for backend, npm run lint:fix for frontend)
• Code is self-reviewed
• No breaking changes
• All CI checks pass