Avoid spawning child processes in different server modes

[conniey]

In "single" and "consolidated" mode, internal tool calls spawn new child processes.

• Make the solution for NamespaceToolLoader (where the problem is fixed) more generalized.

Related #849 #932

[fanyang-mono]

Tools from external MCP server are currently discovered by RegistryDiscoveryStrategy and loaded by ServerToolLoader. And ServerToolLoader spawns child processes.