feat: add Microsoft Learn MCP integration + refine assistant system prompt

- Integrate Microsoft Learn MCP server via Anthropic's MCP connector
  for real-time documentation lookups (docs search, fetch, code samples)
- Refine system prompt: clear persona, structured answer guidelines,
  when to use RAG flows vs Microsoft Learn, guardrails against fabrication
- Add ENABLE_MCP_MICROSOFT_LEARN config toggle (default: True)
- Fix bugs from prior edit: wrong MCP URL, broken indentation, undefined
  usage/token variables, NOT_GIVEN for disabled MCP params
- Log MCP tool usage and cache performance

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/core/config.py

@@ -84,6 +84,9 @@ class Settings(BaseSettings):
     AI_MODEL_GEMINI: str = "gemini-2.5-flash"
     AI_MODEL_ANTHROPIC: str = "claude-haiku-4-5-20251001"
 
+    # MCP (Model Context Protocol) integrations
+    ENABLE_MCP_MICROSOFT_LEARN: bool = True
+
     # Embedding / RAG
     VOYAGE_API_KEY: Optional[str] = None
     EMBEDDING_MODEL: str = "voyage-3.5"