fix(app-server): expose remote MCP servers in plugin read (#26453)

## Why

Remote plugin detail responses include MCP server metadata under
`release.mcp_servers`, but Codex did not deserialize or propagate that
field. As a result, `plugin/read` always returned an empty `mcpServers`
list for remote plugins, so the plugin details pane omitted the MCP
Servers section even when the remote plugin declares one.

This affects uninstalled plugins as well: the remote detail API is the
source of truth and returns MCP server keys without requiring a local
plugin bundle.

## What changed

- Deserialize MCP server entries from remote plugin detail responses.
- Normalize their keys into a sorted, deduplicated list on
`RemotePluginDetail`.
- Return those keys from app-server `plugin/read` instead of hardcoding
an empty list.
- Add regression coverage proving an uninstalled remote plugin returns
its MCP server names.

## Test plan

- `just test -p codex-core-plugins`
- `just test -p codex-app-server plugin_read`

codex-rs/core-plugins/src/remote.rs

@@ -167,6 +167,7 @@ pub struct RemotePluginDetail {
     pub app_manifest: Option<JsonValue>,
     pub skills: Vec<RemotePluginSkill>,
     pub app_ids: Vec<String>,
+    pub mcp_servers: Vec<String>,
 }
 
 #[derive(Debug, Clone, PartialEq)]
@@ -417,6 +418,13 @@ struct RemotePluginReleaseResponse {
     interface: RemotePluginReleaseInterfaceResponse,
     #[serde(default)]
     skills: Vec<RemotePluginSkillResponse>,
+    #[serde(default, skip_serializing_if = "Vec::is_empty")]
+    mcp_servers: Vec<RemotePluginMcpServerResponse>,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Deserialize, Serialize)]
+struct RemotePluginMcpServerResponse {
+    key: String,
 }
 
 #[derive(Debug, Clone, PartialEq, Eq, Deserialize, Serialize)]
@@ -948,6 +956,14 @@ async fn build_remote_plugin_detail(
             enabled: !disabled_skill_names.contains(&skill.name),
         })
         .collect();
+    let mut mcp_servers = plugin
+        .release
+        .mcp_servers
+        .iter()
+        .map(|server| server.key.clone())
+        .collect::<Vec<_>>();
+    mcp_servers.sort_unstable();
+    mcp_servers.dedup();
 
     Ok(RemotePluginDetail {
         marketplace_name,
@@ -959,6 +975,7 @@ async fn build_remote_plugin_detail(
         app_manifest: plugin.release.app_manifest,
         skills,
         app_ids: plugin.release.app_ids,
+        mcp_servers,
     })
 }