server : implement namespaced tools in responses API

[aldehir]

Overview

Fixes #23229

Implement namespaced tools in the Responses API. These tools are commonly used to group MCP servers.

Additional information

Tools names are flattened to ns_{namespace}___{tool_name}, with the expectation that no clients provide a namespace with 3 underscores (___).

On response back, the namespace and name are extracted from the flattened name to fill in the name and namespace fields in the function call.

There are no existing tests for responses tool calls, looking into implementing some.

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: Yes, steered to produce implementation. At first it wanted to propagate namespaces throughout the task, but I opted out of that to maintain a "good enough" approach.