fix: use query params instead of request body for decode_plugin_from_identifier

The decode_plugin_from_identifier endpoint was sending plugin_unique_identifier
in the request body with a GET request. This causes issues with HTTP intermediaries
like Google Cloud Run's frontend, which rejects GET requests with a body as
malformed (returning 400 Bad Request).

Changed from `data=` (request body) to `params=` (query parameters), which is:
- Consistent with similar GET endpoints (fetch_plugin_manifest, fetch_plugin_by_identifier)
- Compliant with HTTP standards (GET requests should not have semantic body content)
- Compatible with Cloud Run and other HTTP proxies/load balancers

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

api/core/plugin/impl/plugin.py

@@ -209,8 +209,7 @@ class PluginInstaller(BasePluginClient):
             "GET",
             f"plugin/{tenant_id}/management/decode/from_identifier",
             PluginDecodeResponse,
-            data={"plugin_unique_identifier": plugin_unique_identifier},
-            headers={"Content-Type": "application/json"},
+            params={"plugin_unique_identifier": plugin_unique_identifier},
         )
 
     def fetch_plugin_installation_by_ids(