Fix/provider deleted display

[mikewoo]

Change Type

• Bug fix (fix)
• New feature (feat)
• Documentation (docs)
• Refactoring (refactor)
• Other

Description

Fix: when a Provider is soft-deleted or disabled, the Agent edit page and Workflow LLM node edit page displayed the raw numeric providerId instead of the provider name.

Backend changes:

• Added PROVIDER_IN_USE / PROVIDER_IN_USE_BY_WORKFLOW error codes; delete now checks for Agent and Workflow LLM node references before allowing deletion
• Agent list/detail endpoints batch-resolve provider names via selectBatchIds, injecting defaultProviderName, defaultProviderType, and defaultProviderAvailable fields
• Workflow LLM node detail endpoint injects providerAvailable and providerName into config JSON

Frontend changes:

• AgentList: provider dropdown shows "Name(不可用)" as a disabled red option for unavailable providers; model input fills "model(不可用)" in red text
• WorkflowEdit: same red fallback for unavailable providers and models
• "(不可用)" / "(Unavailable)" suffix auto-stripped before form submission
• Non-scoped CSS ensures reliable penetration of Element Plus 2.14 component styles

Testing

• mvn test passes locally
• Core functionality manually verified
• New features have test coverage

Checklist

• Database indexes: new queries have index coverage
• Workspace isolation: Service layer queries filter by workspace_id
• Error handling: uses ErrorCode enum
• Log format: pure JSON format, UTC timezone
• Config security: no hardcoded passwords / API keys
• XSS protection: v-html uses DOMPurify.sanitize

Related Issue

Closes #2

Screenshots (if applicable)