refactor: extract MCP tool provider lifecycle

Summary

• introduce MCPToolProvider to own MCP dynamic tool lifecycle state
• route AgentLoop MCP connect, reload, close, and runtime-context behavior through the provider
• update MCP lifecycle tests to assert provider-owned state

Closes #4858

Tests

• PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 PYTHONPATH=. pytest -p pytest_asyncio.plugin tests\agent\test_mcp_connection.py tests\agent\tools\test_self_tool.py -q
• ruff check nanobot\agent\loop.py nanobot\agent\context.py nanobot\agent\tools\mcp.py nanobot\agent\tools\self.py