refactor: move MCP service from api/routes to agent package

MCP is an optional AI feature that already depends on
agent.gns3_copilot (Gns3Connector, nornir/netmiko tools) and whose
MCP_AVAILABLE feature flag lives in gns3server/agent. Moving it there
collocates all AI features under one tree and removes AI code from the
core REST routes.

- git mv gns3server/api/routes/mcp -> gns3server/agent/mcp (no content changes)
- api/server.py, core/tasks.py: update import paths
- tests: tests/api/routes/mcp -> tests/agent/mcp, rewrite patch BASE and
  handler imports; fix MCP_DIR depth in test_tool_params.py
- agent/__init__.py: probe the SDK via importlib.import_module so the
  top-level name "mcp" is not bound in the agent namespace (it would
  shadow the new gns3server.agent.mcp subpackage and break
  'from gns3server.agent import mcp')
- docs: update source file paths

Verified: full suite 1567 passed; 82 MCP tools registered, SSE mounted
at /v3/mcp/transport.

gns3server/core/tasks.py

@@ -97,7 +97,7 @@ async def startup(app: FastAPI) -> None:
     from gns3server.agent import MCP_AVAILABLE
 
     if MCP_AVAILABLE:
-        from gns3server.api.routes.mcp import set_mcp_server_ready
+        from gns3server.agent.mcp import set_mcp_server_ready
         set_mcp_server_ready(True)
     log.info("GNS3 server startup completed")