Use MCP server instructions in deferred namespace descriptions (#21053)

## Why

MCP servers can provide `instructions` that explain what their tools are
for. Directly exposed MCP namespaces already use those instructions when
a connector description is not available, but deferred `tool_search`
results did not preserve that fallback. The direct path falls back from
connector metadata to server instructions, while the deferred path only
carried `connector_description` and otherwise fell back to generic
namespace text.

That meant a plain MCP server could provide useful model-facing guidance
and still appear as `Tools in the X namespace.` whenever it was
discovered lazily through `tool_search`.

## What changed

- Store one model-facing `namespace_description` on `ToolInfo`, using
connector descriptions for connector-backed tools and server
instructions for plain MCP servers.
- Thread that namespace description through the `tool_search` source
list, search indexing, and returned namespace metadata.
- Add an end-to-end regression test for deferred non-app MCP search
results exposing server instructions as the namespace description.

## Verification

- `cargo test -p codex-tools
search_tool_description_lists_each_mcp_source_once --lib`
- `cargo test -p codex-core --test all
tool_search_uses_non_app_mcp_server_instructions_as_namespace_description`

codex-rs/codex-mcp/src/connection_manager_tests.rs

@@ -40,7 +40,7 @@ fn create_test_tool(server_name: &str, tool_name: &str) -> ToolInfo {
         server_name: server_name.to_string(),
         callable_name: tool_name.to_string(),
         callable_namespace: tool_namespace,
-        server_instructions: None,
+        namespace_description: None,
         tool: Tool {
             name: tool_name.to_string().into(),
             title: None,
@@ -55,7 +55,6 @@ fn create_test_tool(server_name: &str, tool_name: &str) -> ToolInfo {
         connector_id: None,
         connector_name: None,
         plugin_display_names: Vec::new(),
-        connector_description: None,
     }
 }