feat: Expose plugin versions and gate plugin sharing (#22397)

- Adds localVersion to plugin summaries and remoteVersion to share
context, including generated API schemas.
- Hydrates local and remote plugin versions from manifests and remote
release metadata.
- Adds default-on plugin_sharing gate for shared-with-me listing and
plugin/share/save, with disabled-path errors
    and focused coverage.

codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.v2.schemas.json

@@ -8838,6 +8838,14 @@
         "remotePluginId": {
           "type": "string"
         },
+        "remoteVersion": {
+          "default": null,
+          "description": "Version of the remote shared plugin release when available.",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "sharePrincipals": {
           "items": {
             "$ref": "#/definitions/PluginSharePrincipal"
@@ -9254,6 +9262,14 @@
           },
           "type": "array"
         },
+        "localVersion": {
+          "default": null,
+          "description": "Version of the locally materialized plugin package when available.",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "name": {
           "type": "string"
         },