feat(api): LLM polling support (#37462)

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>
Co-authored-by: WH-2099 <wh2099@pm.me>

api/core/plugin/impl/exc.py

@@ -5,6 +5,13 @@ from pydantic import TypeAdapter
 
 from extensions.ext_logging import get_request_id
 
+# NOTE: Avoid renaming exception classes in this file, since
+# the `_handle_plugin_daemon_error` in api/core/plugin/impl/base.py
+# build exception instances based on the class name.
+#
+# Renaming of exception classes could result in incorrect exception
+# being raised.
+
 
 class PluginDaemonError(Exception):
     """Base class for all plugin daemon errors."""
@@ -75,6 +82,10 @@ class PluginInvokeError(PluginDaemonClientSideError, ValueError):
         )
 
 
+class PluginLLMPollingUnsupportedError(PluginInvokeError):
+    """Plugin-backed LLM polling is unavailable for the requested model."""
+
+
 class PluginUniqueIdentifierError(PluginDaemonClientSideError):
     description: str = "Unique Identifier Error"