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/tools.rs

@@ -35,16 +35,16 @@ pub struct ToolInfo {
     /// Model-visible namespace used for deferred tool loading.
     #[serde(rename = "tool_namespace", alias = "callable_namespace")]
     pub callable_namespace: String,
-    /// Instructions from the MCP server initialize result.
-    #[serde(default)]
-    pub server_instructions: Option<String>,
+    /// Model-visible namespace description.
+    // Keep the old serialized field name readable for cached ToolInfo values.
+    #[serde(default, alias = "connector_description")]
+    pub namespace_description: Option<String>,
     /// Raw MCP tool definition; `tool.name` is sent back to the MCP server.
     pub tool: Tool,
     pub connector_id: Option<String>,
     pub connector_name: Option<String>,
     #[serde(default)]
     pub plugin_display_names: Vec<String>,
-    pub connector_description: Option<String>,
 }
 
 impl ToolInfo {