fix: real stub class in the missing-package fallback

Runtime regression in the previous simplification — the guardrail
registry iterates every registered class at load time and calls
get_supported_event_hooks(). Fallback of ConductGuardrail = None
crashed the whole registry with AttributeError, which cascaded into
unrelated guardrails' tests (noma_v2, repelloai, hide_secrets,
provider_specific_params, etc.).

Fallback now defines ConductGuardrail as a real subclass of
CustomGuardrail with the required class attrs (SUPPORTED_EVENT_HOOKS
+ get_supported_event_hooks). Matches the pattern the
guardrails_ai integration already uses in the same repo.
raise_if_missing_package still fires before instantiation so users
see the friendly pip install error.

All four budget gates re-verified locally against
upstream/litellm_internal_staging:
  ruff_strict_gate    OK
  test_quality_gate   OK
  type_discipline_gate OK
  type_check_gate     OK

litellm/proxy/guardrails/guardrail_hooks/conduct/conduct.py

@@ -1,11 +1,18 @@
 """Conduct Guard as a LiteLLM guardrail.
 
-Pure alias for the ``conduct-litellm-guard`` PyPI package. The
+Thin alias for the ``conduct-litellm-guard`` PyPI package. The
 ``ConductGuard`` class ships with ``SUPPORTED_EVENT_HOOKS`` +
 ``get_supported_event_hooks`` since plugin 0.2.3, so this file no
 longer needs a subclass wrapper — keeps LiteLLM's type-discipline /
 basedpyright / test-quality budget gates satisfied.
 
+When the standalone package isn't installed we still register a real
+stub class so LiteLLM's guardrail registry can scan
+``get_supported_event_hooks`` at load time without crashing (see
+BerriAI/litellm#38143 CI regression: registry iteration expects every
+registered class to expose the hooks classmethod). Instantiation of
+the stub raises ``ImportError`` via ``raise_if_missing_package``.
+
 Install: ``pip install "conduct-litellm-guard>=0.2.3"``
 Source:  https://github.com/sseshachala/conductai/tree/main/packages/conduct-litellm-guard
 Docs:    https://conductai.ai/guard
@@ -13,21 +20,16 @@ Docs:    https://conductai.ai/guard
 
 from __future__ import annotations
 
+from typing import ClassVar
+
+from litellm.integrations.custom_guardrail import CustomGuardrail
+
 _import_error_message = (
     "conduct-litellm-guard is required for the Conduct guardrail. "
     'Install it with: pip install "conduct-litellm-guard>=0.2.3"'
 )
 
 
-# ── Base plugin import ─────────────────────────────────────────────────
-# Raising ImportError at module load caused the guardrail-hook auto-loader
-# to treat a missing ``conduct-litellm-guard`` as "hook unavailable" and
-# silently drop the registration. Users saw configs load with no guardrail
-# active and no error message. Instead we surface the friendly error at
-# config-load time from :func:`raise_if_missing_package` — called by
-# ``initialize_guardrail`` before construction.
-# (cursor[bot] finding on BerriAI/litellm#38143.)
-
 try:
     from conduct_litellm_guard import ConductGuard as ConductGuardrail
     from conduct_litellm_guard.guardrail import (
@@ -36,18 +38,37 @@ try:
     from conduct_litellm_guard.guardrail import GuardDecision
 
     _import_error: ImportError | None = None
-except ImportError as _import_err:
-    ConductGuardrail = None
+except ImportError as _err:
+
+    class ConductGuardrail(CustomGuardrail):
+        """Stub used when ``conduct-litellm-guard`` isn't installed.
+
+        Exposes the class-level surface LiteLLM's guardrail registry
+        scans at load time — ``SUPPORTED_EVENT_HOOKS`` +
+        ``get_supported_event_hooks`` — so the registry doesn't crash
+        when this hook is discovered without the runtime dependency.
+        ``initialize_guardrail`` calls ``raise_if_missing_package``
+        before ever constructing this class, so users see a friendly
+        ``pip install`` error rather than the stub silently activating.
+        """
+
+        SUPPORTED_EVENT_HOOKS: ClassVar[tuple[str, ...]] = ("pre_call",)
+
+        @classmethod
+        def get_supported_event_hooks(cls) -> list[str]:
+            return list(cls.SUPPORTED_EVENT_HOOKS)
+
     ConductGuardrailBlocked = None
     GuardDecision = None
-    _import_error = _import_err
+    _import_error = _err
 
 
 def raise_if_missing_package() -> None:
     """Called by ``initialize_guardrail`` before constructing the class.
 
     Surfaces the friendly ``pip install`` error at the actionable moment
-    (config load) rather than silently dropping the hook at module load.
+    (config load) rather than silently dropping the hook or letting the
+    stub run.
     """
     if _import_error is not None:
         raise ImportError(_import_error_message) from _import_error