feat: Support remote plugin list/read. (#18452)

Add a temporary internal remote_plugin feature flag that merges remote
marketplaces into plugin/list and routes plugin/read through the remote
APIs when needed, while keeping pure local marketplaces working as
before.

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.schemas.json

@@ -10965,7 +10965,14 @@
             "type": "string"
           },
           "marketplacePath": {
-            "$ref": "#/definitions/v2/AbsolutePathBuf"
+            "anyOf": [
+              {
+                "$ref": "#/definitions/v2/AbsolutePathBuf"
+              },
+              {
+                "type": "null"
+              }
+            ]
           },
           "mcpServers": {
             "items": {
@@ -10986,7 +10993,6 @@
         "required": [
           "apps",
           "marketplaceName",
-          "marketplacePath",
           "mcpServers",
           "skills",
           "summary"
@@ -13356,7 +13362,14 @@
             "type": "string"
           },
           "path": {
-            "$ref": "#/definitions/v2/AbsolutePathBuf"
+            "anyOf": [
+              {
+                "$ref": "#/definitions/v2/AbsolutePathBuf"
+              },
+              {
+                "type": "null"
+              }
+            ]
           },
           "shortDescription": {
             "type": [
@@ -13368,8 +13381,7 @@
         "required": [
           "description",
           "enabled",
-          "name",
-          "path"
+          "name"
         ],
         "type": "object"
       },