Fix empty tool descriptions

[shijie-oai]

Summary

• Ensure direct namespaced MCP tool groups are emitted with a non-empty namespace description even when namespace metadata is missing or blank.
• Add regression coverage for missing MCP namespace descriptions.

Cause

Latest main can serialize a direct namespaced MCP tool group with an empty top-level description. The namespace description path used unwrap_or_default() when tool_namespaces did not include metadata for that namespace, so the outbound Responses API payload could contain a tool like {"type":"namespace","description":""}. The Responses API rejects that because namespace tool descriptions must be a non-empty string.

Fix

• Add a fallback namespace description: Tools in the <namespace> namespace.
• Preserve provided namespace descriptions after trimming, but treat blank descriptions as missing.

Issue I am seeing

This is what I am seeing on the local build.

[shijie-oai]

I am not really sure what we should do if there is a missing description but it does not like empty string here for sure. Not opposed just to put the name of the tool in here instead but I think the current approach is fine.