fix(mcp): sync manage_mcp enable/disable with live manager

[cyq1017]

Summary

When enabling or disabling an MCP server via the manage_mcp command/action, the configuration was updated in the database but did not take effect on the active MCPManager instance. This PR connects/disconnects the live MCP server dynamically from do_manage_mcp so that changes take effect immediately without needing to restart the application.

Target branch

• This PR targets dev, not main.

Linked Issue

Fixes #3824

Type of Change

• Bug fix (non-breaking — fixes a confirmed issue)

Checklist

• I searched open issues and open PRs — this is not a duplicate.
• This PR targets dev
• My changes are limited to the scope described above — no unrelated refactors or whitespace changes mixed in.
• I actually ran the app and verified the change works end-to-end.

How to Test

1. Add an MCP server to the database.
2. Run do_manage_mcp with action="disable" and the server_id. Verify that tools registered by this server are dynamically removed from the active tool registry.
3. Run do_manage_mcp with action="enable" and the server_id. Verify that tools are dynamically re-registered.
4. Run the unit tests to verify database commits and connection calls are triggered in correct sequence:
   python3 -m pytest tests/test_mcp_reconnect_args.py

Visual / UI changes — REQUIRED if you touched anything that renders

N/A (Backend logic only, no UI/visual changes)