test: Add legacy auth handlers and regression tests

- Add legacy/ directory with preserved function-based auth handlers
- Add test_regression.py comparing legacy vs new handler responses
- Ensure backwards compatibility during migration period

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

Author: kwonkwonn

changes/7865.enhance.md

@@ -0,0 +1 @@
+Migration of `manager/api/auth` into pydantic model. which include DTO, test, and Handler pattern.

src/ai/backend/manager/api/auth/__init__.py

@@ -9,13 +9,16 @@
 
 from __future__ import annotations
 
+from typing import Any
+
+from aiohttp import web
+
 from .decorators import (
     admin_required,
     auth_required,
     auth_required_for_method,
     superadmin_required,
 )
-from .handler import create_app
 from .middleware import auth_middleware
 from .utils import (
     _extract_auth_params,
@@ -24,6 +27,25 @@
     validate_ip,
 )
 
+
+def create_app(
+    default_cors_options: Any,
+) -> tuple[web.Application, list[Any]]:
+    """Create aiohttp application for auth API endpoints.
+
+    Note: This wrapper defers handler import to avoid circular imports:
+        dto.context -> services.processors -> repositories.repositories
+        -> repositories.session.repository -> api.session -> api.auth/__init__
+        -> api.auth/handler -> dto.context (circular!)
+
+    Root cause: repositories.session.repository imports api.session.find_dependency_sessions
+    TODO: Move find_dependency_sessions to repository layer, then remove this wrapper.
+    """
+    from .handler import create_app as _create_app
+
+    return _create_app(default_cors_options)
+
+
 __all__ = (
     # Middleware
     "auth_middleware",

src/ai/backend/manager/api/legacy/__init__.py

@@ -0,0 +1,15 @@
+"""
+Legacy API handlers.
+
+This module contains old function-based handlers preserved for regression testing.
+These handlers should NOT be used in production - use the new class-based handlers instead.
+
+Contents:
+- auth: Legacy auth handlers (replaced by api/auth/ package)
+"""
+
+from __future__ import annotations
+
+from . import auth
+
+__all__ = ("auth",)