Expose workspace plugin publish capability (#35254)

## What changed

- Add nullable `canPublishToWorkspace` metadata to plugin share contexts and `plugin/share/save` responses.
- Preserve the remote capability through catalog parsing and app-server responses so clients can decide whether to offer workspace-directory publishing.
- Document that clients should fail closed when the capability is unavailable.

## Testing

- Cover remote catalog parsing, share-save response propagation, and protocol serialization.

GitOrigin-RevId: b0346b3b330169838f800dfdf21dfa5db1a47f4d

codex-rs/app-server-protocol/src/protocol/v2/tests.rs

@@ -3701,11 +3701,13 @@ fn plugin_share_params_and_response_serialization_use_camel_case_fields() {
         serde_json::to_value(PluginShareSaveResponse {
             remote_plugin_id: "plugins~Plugin_00000000000000000000000000000000".to_string(),
             share_url: String::new(),
+            can_publish_to_workspace: Some(true),
         })
         .unwrap(),
         json!({
             "remotePluginId": "plugins~Plugin_00000000000000000000000000000000",
             "shareUrl": "",
+            "canPublishToWorkspace": true,
         }),
     );