fix(ai): stop advertising semantic_search when no vector search plugin is enabled

[lb1038678031]

Fixes #1532.

Problem

The semantic_search MCP tool was always advertised to the model, even when
no Vector搜索 plugin was enabled. The model could select it, the call
reached EmbeddingService.搜索Similar, and every such conversation logged
semantic search failed: semantic search is not available: no vector search plugin is enabled — polluting logs and wasting a tool-call round trip on a
capability that does not exist in the deployment.

Fix

• EmbeddingService.Available() reports whether a Vector搜索 plugin is
  currently enabled.
• MCPController.Semantic搜索Available() exposes that to the AI chat.
• getMCPTools() omits the semantic_search tool when unavailable; all
  other MCP tools keep working exactly as before.
• The default AI prompts (zh/en, and admin-custom prompts) drop the
  semantic_search instruction lines in the same situation, so the model is
  no longer nudged toward a missing capability.

Testing

• 新建 unit tests: tool advertisement with/without the capability, prompt-line
  stripping, and prompt adaptation (internal/controller/ai_tools_test.go).
• go test ./internal/controller/ — all pass.

[LinkinStars]

Thank you, @lb1038678031, for this thoughtful and well-tested fix! It not only prevents the unavailable tool from being advertised, but also keeps the prompts aligned with the deployment capabilities. Much appreciated!