Fix empty tool descriptions (#17946)

## 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.
<img width="1593" height="488" alt="Screenshot 2026-04-15 at 10 55 55
AM"
src="https://github.com/user-attachments/assets/bab668ba-bf17-4c71-be4e-b102202fce57"
/>

---------

Co-authored-by: Sayan Sisodiya <sayan@openai.com>

codex-rs/tools/src/lib.rs

@@ -85,6 +85,7 @@ pub use responses_api::ResponsesApiNamespace;
 pub use responses_api::ResponsesApiNamespaceTool;
 pub use responses_api::ResponsesApiTool;
 pub use responses_api::ToolSearchOutputTool;
+pub(crate) use responses_api::default_namespace_description;
 pub use responses_api::dynamic_tool_to_responses_api_tool;
 pub use responses_api::mcp_tool_to_deferred_responses_api_tool;
 pub use responses_api::mcp_tool_to_responses_api_tool;